It can be in any Fallout games. Mine's when I was doing the side quest "A Final Plan for Esteban" in FNV for the first time around 5 years ago. For those of you who don't know or forgot the quest, it's where Private Morales asks me to retrieve the corpse of her dead husband. It was going well until I found his corpse because I didn't know where to go, I know it's stupid because all I need do is bring the body to the two NCR soldiers that is not too far from where you pick it up, but somehow I missed them and bring the corpse to NCR Ranger safehouse and then Camp McCarran, but the quest is still uncompleted and I thought the game is bugged. So I googled it and realized how stupid I was, then I bring the body from Camp McCarran back to REPCONN headquarters and finally completed this goddamn quest. In total, I think I spend at least one and half hours, if not, more, to complete this. This memory stuck in my mind even till this day and I facepalm everytime when I do that quest again and ask myself "How can you be this dumb?"

When I first played FNV it was on PS3. I went to novac and my friend was next to me showing me all the cool places. And we started dicking around laughing our asses off killing people with mister sandman. And it I killed the old vertibird pilot. And later on I wanted to get the Gannon Family armor. But I found out that I failed the quest a long time ago. So I was mad and felt silly, but it was worth it.

On my first ever playthrough I tried heading north to New Vegas from Goodsprings through the Cazadors, I tried for so long and was furious at how difficult it was...

Got a mod that gave me access to an orbital missile satellite. Tested it and promptly blew myself and most of Novac into pieces.

Stupidest thing I've done, probably shooting a fat man in the metro at some ghouls that were running at me haha didn't realise they were as close as they were.

Run right through the cazador areas of New Vegas, definitely. Oh wait....actually....thinking that my mod load order was completely stable :p

First ever playthrough of FO3
(my first ever FO game too)

"what's that? A deathclaw, yeah I can take him."
*pulls out 10mm SMG*

needless to say, I didn't make it...

Use the machine as a shield, I think you can guess what happened further)))

Charging at ghouls with a machete, thinking they would die like the zombies in Dead Island and L4D2.

I shot Shaun (Father) with a Gauss Rifle the moment I saw him. Maybe dumb, but hey if he calls his mom "collateral damage" then he better die.

Don't really have alot of mistakes to regret over my Fallout experiences. Even though, I test all outcomes of 'social experiments'. But the biggest mistake in Fallout New Vegas I ever made (And I know I said this hundreds of times already) as far as I can recall, is trusting the NCR. They were a shit ally, and now I will never side with them again.

Dragging the dead alien from the saucer in fallout 3 all the way back to Megaton just so I could put him in the bathtub outside of Megaton, then dropping a bunch of empty whiskey bottles around him like he was drunk...

I thought I was so cool...

Try to take out a Deathclaw using a razor to get the GRA challenge on Very Hard + Hardcore without realizing that I could just use another weapon to lower down its health until I could kill it with the weapon needed for the challenge.

Modded Fallout 4 for like 3 weeks straight, had over 200 mods, fixing crashes, fps, etc. Then I played my heavily modded game for 2 days, got bored and never played again. lol

Not checking for all open doors before I ignited the gas leak in Vault 22 (I was so surprised).

trying to go through new vegas through sloan and trying to kill the deathclaws with a varmint rifle with low ammo. this was back when i played it on the 360 (side note i played the game long enough to make it to the year 2286) and i had no memory card so i had to restart the game over and over again also trying to kill the van graffs which failed miserably quite possibly the stupidest thing I've ever done in fallout.

Probably forgetting to save before putting some mines in someone's pocket in Megaton in FO3. 10 year old me was pretty confused

Hunting deathclaws with an old 9 iron.

Using only one save slot the entire game before heading to the glow in Fo1.

Killing Mr. House but not thinking better about it, i wish i had sided with him