For me, it was when I accidentally found the Hard Luck Blues Quest when I was just a level 9. I thought that since I was able to unlock that quest it was supposed to be easy enough for me to handle. I go down to Vault 34 with Boone as my only companion...and...and it gets fuzzy after that. I got lost about 147 different times I think, got advanced rad sickness THREE different times, used all of my 32 Rad-Aways, amd just hated my Fallout life for a couple days because of that misery (I was way too stubborn to use a Youtube walkthrough) because it took me like four different sittings to finally finish that BS. Blahhhh.
Let's hear some more stories of your worst Fallout decisions.

Play a EW character pre-patch on the 360... It was frustratingly boring.
Oh and I saved over my first EW character with my second EW character by mistake.
Wasn't really raging about that though, was more like "*Sigh*" and just kept on going with the new one.

I gave one of my companions grenades.

My entire body regretted it in about ten minutes flat.

Ha ha better than asking them to hold a missile launcher and some missiles.

How bout overwriting your lvl 30 character after completing the BOS quest line cause you were responding to some silly topic on this forum :facepalm: Now I'm back to lvl 27 and have to find those damn air filter componets AGAIN. ahhhhhh. I might just abandon this character and go back to oblivion for a sec.

I was still neutral with the Legion until I fast-travelled to the fort with Boone as a companion...oops, reload. And I killed House before turning in the snowglobes in my inventory.

Try to shoot Cazador in the head?

I found it fun and challenging in vanilla on PC before patch (but admittedly not "right")?

Those air filter cartridges are a PAIN to get, too... lots of mazework and headaches.

Timing a plasma projectile is easier with a mouse compared to clunky anologue sticks.

Trying to kill Caesar with Benny.

Why would you use plasma pre-patch?

Whats the most stupid thing I've ever done? Ummm... lets see. Killed Mr.House, killed BOS, killed NCR, killed an inocent, killed (list continues, you lose intrest...)

holy hand grenade, 3 ranks of demo expert, splash damage, rauls head in the way.

Playing for about 35 min without saving I guest ! <_< PS3

I've been going through this with Cass with the dynamite and Boone with the grenades. Look out Courier!

+ Raul on the Heavy Incinerator? Look out ED-E! He was in flames a few times.

Trying to sneak through a quarry loaded with Deathclaws at Level 8, with one of my legs crippled, no stimpaks and one stealth boy. The stealth boy literally ran out just as I was sneaking past the Alpha Male. Next thing I know my head's in another continent and my headless corpse is spattering blood everywhere.

Running from a large group of Cazadores I'd startled before I knew how dangerous they were, kept taking potshots at them and downing stimpaks until finally I'd almost killed the last one...then while I was firing I ran backwards off a cliff...that was literally only about three feet high. I found out just how quickly my kneecaps can exit through my shoulders at low health. >_>

There are far more embarrassing [censored]-ups...like accidentally stealing from the Van Graffs while trying to talk to Gloria because I wasn't paying attention. Still I ended up being the prettiest pile of goo in the store.

Multiplas and Plasma Caster were the only weapons doing any damage towards deathclaws, super mutants and desert rangers.
Had to use them for for stronger enemies.
Then I found Gauss Rifle.
Then I abused Gauss Rifle.
Then I stopped using Gauss Rifle.
And back to boring again.

Pissing off the legion at lvl 4, [censored] your assassins caeser.
Startin the game I saw the strip, and signs saying "danger deathclaws" then thinking to myself "how many could there be ? and I could outrun them". Yeah stupid idea, had to walk around.
And the first time you meet cazadors "oh look a few bugs, this will be easy".

All the above.

My bigest mistake however is playing it so damn much with every type of character.
That I now rarely make mistakes and start to plan ahead without meaning to.

I was a level 4, Guns and Explosive character. I saw a man being attacked by some creatures, I go to help him and find out they are Giant Radscorpions. I refrain from using my Dynamite because I would likely kill the guy I was trying to rescue. So I ran in with the only weapon I thought would damage them (3), the Shotgun. Lets just say it failed, their armour completely negated my attacks, and after they killed the Prospector I was forced to run for my life. I had to run all the way to the Mojave outpost controlled by the NCR, as they would aid me. My Leather armour had been destroyed by the ferocious attacks of the RadScorpions and I stupidly put on the only other set of Armour I had with me... My Legion armour... I got to the gates of the Outpost with half health left and No Stimpacks. But then the NCR started shooting me too. I was Dead in a few seconds... Caught in a crossfire of Scorpions and NCR troopers. They attacked me because of the outfit I HAD to put on to save my life.

I learnt a few things that day. Like not saving anybodies life XD.

Got a new one, take a 12.7 smg which has very bad accuracy even at point blank range, add in ED-E..
Then do a combat run of a warehouse full of explosive to laser ants and artillary shells that kill if hit.

Add to this giving Raul too many guns and ammo for them causing a looping CTD everytime he tried to initiate combat.

Accidently stealing a pencil in the brotherhood bunker. I didnt even know why they were shooting me

Getting wasted IRL then going to Camp McCarren only to blow everyones face off then save over my main file! Whoopsie! Def went wild card in that run through

I usually play carefully in order to avoid doing something stupid, but last night while doing a Legion run my actions near the end probably qualified for a Darwin Award.

I was at the point where I was to go to the Dam and frag Kimball, so I put on the NCR armor to avoid getting shot to bits while setting up. I get to the damn, and, even though I knew they would react violently to pulling a weapon from my other runs, I pull out my AMR in full view of two Rangers before crouching down into Stealth mode :facepalm:. Needless to say, six NCR people opened up on me at once, and even with my DR perks and Implant that wasn't going to end well. Cue reload to the meeting with Cato Hostilius.

So, I try again, and this time I bring a Stealthboy to make sure they don't see me when I fire. I know they will afterward, since it's not silenced, but I also know I just have to get a short distance from the visitors' center entrance and I can FT away regardless, so I set up accordingly. That part goes well, although I probably should have waited fully out of sight for a few RL seconds before popping the SB so I would have been 'fully' invisible while shooting, and I make my way to the road a couple of hundred feet away and FT out to The Fort. So far, so good, right? WRONG. As soon as I 'zone in', all 5 guys at the gate open fire immediately even though they should be friendly. I'm wondering WTF set them off, then I consider the fact that they usually accost you at that point and take your weapons and chems and conclude that attempting to bypass that pisses them off. So I try again, this time FTing in while visible, and they go off on me again. Then it hits me: I forgot to take off the NCR armor I was wearing to do the hit, even though I know it will make them auto-hostile. :facepalm: :facepalm:

(Note to self: do not attempt any more faction assassinations at 0400 or so after a marathon session, and bring a damn silencer next time)

Heh heh that's a great story.

Mine, I was completely loathed, hated beyond hated by the NCR from primm, with a melee and explosives character.
I managed to sneak into the middle avoid all troubles and even set up the mission in the non combat way despite the dogs and rangers all around with a sneak of 50ish.

Got so bored waiting with my psycho build I pulled out a missile launcher and from the far end of the entrence fraged him in one shot.

I tend to have little patience.

my worst was probably going death claw hunting killing pretty much all of them in the quarry and the mother and the alpha male then not saving and my ps3 froze. [hint] if on ps3 ALWAYS SAVE