I'm compiling a list of various bugs that have been discovered. Not vague ones like "excessively crashy" or stuff like that, but discrete bugs whose symptoms can be concisely described, or have been diagnosed by someone who knows the workings. Hopefully, with a centralized list of bugs, Obsidian will be better able to locate the bugs themselves and fix them.

There are three categories: Executable-level, parts which are most likely involved in the core FalloutNV.exe file, Module-level, bugs that exist in the main module of the game, the FalloutNV.esm file, and Asset-level, which are the various NIF 3d models, DDS textures, XML textures, and etc.

Optimally, if you have a repoty yourself, it'd be best to have it already diagnosed. If not, please list it only if it has conditions you can reliably repeat. Be thorough. The more control you have in anolyzing the symptoms of a bug, the easier it is to ascertain the cause.

Asset-level bugs seem to be beyond the purview of fixing via patches, from the lack of such fixes so far in patches. I wouldn't count on fixes on them via patches.

Executable-level bugs:

• Non-hitscan projectiles do no base projectile damage if they have an explosion on impact. For instance, the 400 extra damage the the Fat Man supposedly does on any actor it directly hits is not applied, only the 600 damage from the explosion.
  
• Objects with environmental reflection shader effects (like the Plasma Rifle) do not reflect total lighting in their enviro reflection effect while in firstperson, only ambient lighting in the cell. Thirdperson is not affected. This bug only manifests when the mesh's enviro map settings are set to react to lighting (Unknown Int 2 in BSShaderPPLightingProperty set to 32769 when using NIFSkope). This bug existed originally in FO3.
  
• Weight-reducing weapon mods do not apply their weight reduction effect when they're equipped. A Fat Man Little Boy is 15 pounds when just in the pack, but shoots back up to 30 pounds according to the total encumbrance value when equipped.
  
• Loose files associated with others will not load. Only files packed in a BSA will load. For instance, EGM files do not load for any headwear items unless they are packed in a BSA. It's likely that EGTs, TRIs, and any other such files associated with the FaceGen system may be affected.

Module-level bugs

• "Always use World Orientation" is not checked for the Holy Frag Grenade explosion, resulting in the mushroom cloud coming off at funny angles not perpendicular with ground level.
  
• For Auld Lany Syne is rife with bugs, see http://www.gamesas.com/index.php?/topic/1196156-fonv-compiled-bug-list/page__p__17780854#entry17780854
  
• Arcade Ganon will not be re-recruitable unless your nonhuman companion (ED-E, Rex) is dismissed too. No dialog option even appears to hire Arcade if you have a nonhuman companion with you.
  
• Boone head tracking/weapon holstering in combat bug, caused by excessive package evaluation when not needed. http://www.gamesas.com/index.php?/topic/1196156-fonv-compiled-bug-list/page__view__findpost__p__17780891

Asset-level bugs:

• On the iron sight aim for all non-pistolgrip rifles (like the Hunting Rifle), the animation is significantly off-center, making aiming with ironsights difficult. The bug can be demonstrated on the Hunting Rifle; duck down and aim down iron sights to get maximum accuracy while in third person. While still crouching and aiming, switch to first person view. Note that the crosshair still persisting in first person shows that it's not lined up with the front sight bead. A mod for PC fixes this: http://www.newvegasnexus.com/downloads/file.php?id=38927
  
• Several other weapons have misaligned sighting nodes.
  
• Marcus's normal map has a backwards vertical (green) channel.
  
• 44 Magnum Revolver HD Frame causes serious issues with the speed loader and animations in general. Might be an executable-level issue, considering that re-jigging the mesh directly from the supposedly proper-functioning base NIF produces the same bugged results. However, because issues with animations on the .44 Magnum existed in Fallout 3, it may be asset-level as well since other revolvers added by Obsidian do not have these problems.
  
• Sound tags are missing in several of the walk/run animations for 2handhandle weapons. The game engine will only play the basic footstep sounds, and none of the weight-related armor footstep sound effects.

If you list one, remember: already have it diagnosed, or be as thorough as possible in describing the symptoms. Vague ones won't be listed.

THIS IS NOT A PLACE TO RANT ABOUT BUGS. If you do, your post will be reported for being off-topic, even if the content of your post is also reporting a bug. List bugs you can describe to the best of your ability, or that you have diagnosed to the root cause of the bug.

Well this is gonna be a mighty list.

I would say that For Auld Lang Syne is the most buggiest quest. Being that it has many bugs do do with it. It may not even be able to be dont on some peoples games.

Bugs:
It is possible to not be able to activate the quest for some unknown reason, despite attempting all trigger options, if you're traveling with ED-E in your party get rid of him and the quest should work.
xbox360 Even if you do not have ED-E as a companion, it is possible to not be able to activate the quest for some unknown reason despite attempting all trigger options.
pc ps3 xbox360 When recruiting Whitman in the hotel room, as soon as she agrees, Arcade will despawn, but will reappear once you leave the room, allowing the player to receive the conversation affirming the checkpoint afterward
pc xbox360 If the player is given a set of Remnant Power Armor and Helmet and already has another set in their inventory, they may appear as one set in the player's inventory, but can be dropped or stored twice.
xbox360 Player may be unable to leave the briefing room at the end of the quest.

Solution: Pickpocket the key from Orion Moreno, if he is still alive (be sure to pickpocket the key from him AFTER he has walked back into the briefing room, or else it will be removed.)
Solution: Kill all the remnants. Fails the quest.
Solution: Reload the autosave from entering the room. Shoot at the remnants as soon as the game loads, which prevents the key from being taken. Fails the quest.
Solution: Convince Moreno to go back in the Briefing Room, finish the quest and then kill the remnants.
Solution: Unlock the door that you entered through first. Then enter the room that the Remnants are in. Shoot Judah with a low damage weapon (BB Gun) as soon as you enter the room (it is helpful to keep firing through the loading screen). The Remnants should become hostile towards you (not Arcade). This prevents Judah from taking the key. Exit the room again so you are in the room with the vertibird and wait for Judah to chase you. Quickly go into sneak mode and use a Stealth Boy, the Remnants should now be friendly again. Talk to Judah and he will take the key and complete the quest. WARNING: It is advisable to leave your companions outside of the bunker (including Arcade) as they will try to kill the Remnants when you shoot Judah.
Solution: If you are doing the mission for the armor you can just use the override key to just go out the hanger door instead of going into the control room of no return, Also if you picked for arcade to not help out at Hoover Dam he will pop out outside and give you the armor (Hope this helped in any way)
pcThis can be fixed through the console by typing: set "16117f".nProFollowersCount (if you want him to stay with the Followers) OR nProEnclaveCount to 3(at least!!).

ps3 Upon entering the Remnant's Bunker, Daisy Whitman might randomly die, causing the mission to fail.
pc xbox360 Sometimes, Orion will appear near Silver Peak Mine after storming out of the bunker and will stand there until the Battle of Hoover Dam.
ps3 xbox360 If Guess Who I Saw Today glitches and can't be completed, Doctor Henry cannot be recruited.
pc xbox360 When the Enclave Remnant appear during the fight at the Hoover Dam, if the player is sided with Mr. House or Yes-Man, the Remnant might be hostile.
xbox360At the end of the quest, Arcade Gannon might not ask about his decision whether to fight or not, and might stop following. His companion wheel might also become inaccessible.

ps3Arcade will not ask about decision but he will follow you.
A possible partial fix is to take Gannon back to the Lucky 38 (or the Old Mormon Fort but I haven't confirmed that). Once there dismiss any non-humanoid companion (Rex or ED-E) and then dismiss Gannon. You might then be able to recruit Gannon once again as a normal follower. (tested on 360)

xbox360 Dismiss any non humanoid (ED-E or REX) and talk to Gannon again. He should follow you then.
ps3 xbox360 Sometimes when you try to get Arcade to rejoin you, there won't be an option in the dialog to do so.

Sometimes asking Rex or ED-E to leave the party can fix this.

ps3 If Arcade and Ed-E are in the hotel room and Daisy is lying on the bed when you enter, talking to her about the reunion can cause Arcade Gannon to disappear.
ps3When leaving the control room after finishing the quest 3 Great Khans might appear.
ps3The player might randomly get another set of remnant power armor at the end of the quest.
ps3If the player got the power armor training from the BoS, there will be no other option but to tell Kreger that he/she already got the power armor training and therefore the player will not get the Remnant power armor from Daisy. The quest ends after leaving the Remnants bunker.
xbox360 It is possible for Dr. Henry to not be present in Jacobstown at all, making it impossible to recruit all members.
xbox360Sometimes when you get the last step to finish the quest, when you exit the command room, the quest will not finish. This can be fixed by exiting the Bunker, and entering again.
xbox360Sometimes Arcade will appear at the exit of the Remnants Bunker in his full Tesla Armor and wander in and out of the bunker.
pc After convincing Moreno to assist the NCR, Arcade will ask if he should go back to Freeside or assist in the battle at Hoover Dam. Upon leaving, he will give the player three stimpaks and his ripper. Reloading a save just before this dialogue, it seems completely random whether or not the ripper is given (even when choosing the same dialogue options every time) - resulting in the player possibly not receiving the weapon.

Note:Bug list is used from wikia. I may add others later.

Also this is a new bug that i experienced. I had Arcade as my Companion and i was in the middle of doing For Auld...(etc.) and i went over to the Jakobstown to do the quest Guess who i say today and I told Arcade to go back to the Lucky 38 because i was going to use lilly for the time being to be used as my companion so i go into the caves..etc. and do the stuff required. So then i went back to the lucky 38 and told lilly to go back to jacobstown and when i got there there was no dialogue for Arcade to join my squad.(Lilly was gotten rid of so there was still room for a huminiod companion). So after about 15 i said to Rex to go back to the lucky 38(where i was at). Then the dialogue for Arcade showed up for him to be my follower again. It was the strangest bug ever. Then i added back rest and it was good as normal..

BUG introduced in Boone's Script last December, not fixed in last patch. Josh Sawyer has said he's sent the info to Bethesda but since I and many others have been posting about it since last December and nothing was done, here it is again.

Craig Boone keeps holstering his weapon with an enemy right in his face. He has mostly stopped headtracking.

On this thread, you can see it at post #37, 38 and the links I posted on that thread lead to the Craig Boone Warn Fix mod that will fix him if you're on PC. If not, he's still broken. I'm waiting for the fix on my PS3.

http://www.gamesas.com/index.php?/topic/1190149-huge-problem-with-bethesda/page__st__20
Kicenna, on 15 May 2011 - 09:41 AM, said:
Josh, I have to ask who'll be working on fixing what's wrong with Boone.

J.E. Sawyer on 15 May 2011 - 10:43 AM, said:
I forwarded those links to Bethesda folks. In the past, every thread I've seen where someone has complained to me about Boone being broken has simply suggested that Boone is broken without articulating what, exactly, the problem is.

This is the fix.
"After the 1.2 update to the game, people were complaining about Boone's behavior - head tracking was not working, and combat was messed up as Boone would holster his weapon constantly. I spent quite a bit of time investigating this, as Boone is a cool guy

This mod fixes an issue in the VDialogueCraigBooneSCRIPT. There is a section that monitors the player's reputation with the NCR and triggers a force greet if certain conditions are met. The code was causing Boone to evaluate his packages every five seconds even when not required. Two semicolons fixes it. On hindsight, it's pretty obvious what was happening. An EVP will cause an NPC to drop out of combat - holster their weapon. When run constantly, apparently it breaks head tracking in the follow packages also."

http://www.newvegasnexus.com/downloads/file.php?id=38842