I've completed many different playthroughs in New Vegas and searched a lot of locations, yet every time I play a new one, I discover sth. new. What were some memorable things / events that happened to you on some later playthrough (not the 1st or 2nd one) that haven't happened to you before? I'm mostly thinking about things / events that you should have discovered earlier because you were in some location for 100 times but simply didn't.

In my current & few recent playthroughs I...:
-... discovered that there's a repairable generator in Sloan, it seems I always missed it before (I'm also surprised that fallout wiki doesn't mention it).
-... knew there was a gun locked in safe in Primm but I never returned to get it in my previous playthroughs, because I had powerful weapons already when I got my lockpick skill to 75. So this time I returned and picked the Lucky up.
-... didn't know that high NCR rep can be used to persuade Henry Jamison to quit caravan business
-... found dead gambler near SS HQ carrying a revolver and a letter

I'll add more when I remember.
Post yours if you want

Never got the enclave armor from the deathclaw unmarked location. But I'd imagine that was something that was hard to do.
Missed the Thorn. Never found it even though I went to the wiki to search it up. I eventually found in in my third play through.
Also didn't find the aliens. I knew it was north of the farm but I had the farms confused and eventually gave up looking for it. Found it in this current playthrough.

I don't think I missed anything obvious other than the FotA would give me a safehouse. In fact, I never got a safehouse in my first or second playthrough. Not my third either. It was only after playing Skyrim and returning to FNV that I ever got a safe house key. I always thought that the only way to get the black ranger armor was from killing one.

-Lucky Shades
-Legion safehouse(free stealth boys)
-having the FotA support the NCR for the final battle
-doing Raul's quest(it didnt trigger)
-fighting Lupa
-Powder Ganger quests
-deathclaw promontory & dead wind cavern(I was too scared to go in either)

Allen Marks and the Pew Pew? I'm just curious

No, he is in one ruined building near SS HQ, it's one of those red-brick buildings.

P.S.
Description from wiki:
" In a red rubble building directly east of the http://fallout.wikia.com/wiki/Monte_Carlo_Suites, a dead gambler with a .357 Magnum has the letter"

I found him. I can't believe I didn't find him until now. I'm so ocd about picking up everything I can find

I've NEVER found that dead gambler :S

It took me 3 playthroughs to stumble upon Aerotech, Camp Guardian and Techutticup mine.

I accidentaly spotted him from very far away when sniping with Varmint rifle to check if there were any Fiends near.
I also can't believe I was always missing out that Sloan generator.

Mr. Cuddles whereabouts. Came across Mr. C while looking up teddy bears in GECK one day and learned his location. He's alive, Lindsay, and Dala didn't lobotomize him!

- Lucky Shades and Legion Safe House (trying to get them this time)
- Helped Powder Gangers take over Goodsprings (felt bad, scrapped the character and started over LOL)
- Helped Powder Gangers defend the NCRCF, when NCR Troopers stormed it. (No negative NCR rep for that one, but those troopers are tough, let me tell you! lol)
- Met the people who were locked up in Vault 34 (freed them during my 2nd play through)
- Oh and finally figured out how to set my hotkeys during my 2nd play through LOL

Found Little Buster dead next to the Mormon Fort
Found the Crashed Vertibird

I'll have more tomorrow.

Almost everything.