I dont know if this qualifies but that would be: buying, installing and playing Fallout 4. Massive waste of time and money. Not to mention broken hopes....

Using the Prototype Medic Power Armour. It’s good if you have an endless supply of stimpaks, but I didn’t have that many stimpaks after walking into Old Olney (deathclaws), so it would just end up wasting my stimpaks before I’d actually need them, I had a lot of food I could eat too instead of using them, so I kept getting killed by the deathclaws and so that was the end of that save game.

Spent twenty minutes on Fallout blowing myself up constantly with the dynamite because I didn't realise I was keeping it in my pocket each time I set the timer.

FO3: Coming to megatone and blowing it up without exploring it.

FNV: Messing with BOS but forgot the fact that i had turned-of the auto-saves .... 4 hour session wasted in one bullet. And Obviously the amount of time i took to get back were i was with the Déja Vu feeling.

F4: Going to Spectacle Island real early in the game, yup mirelurks got me many times until i gave up and came back with heavy artillery. (survival mod)

Going to Quarry Junction when I first played New Vegas back on the 360. It didn't end well.

Trying to build a settlement on a bridge -- I was so deep in concentrating on placing pieces, that I walked backwards off the edge of the bridge and died. I reloaded, and somehow... did the same thing again.

Installing hundreds of mods, without reading their readmes, for my first playthrough of FNV. New Vegas was also my first Fallout game, fortunately, so I had no idea what was considered normal.

I proceeded to The Strip by travelling through quarry junction killing deathclaws with the weathered 10 mm pistol from the preorder pack. I didn't realise that by doing so I was ruining the balance of the early game by abusing Project Nevada's bullet time and damage multiplier mods.

The best part was the looks on my friends' faces when I told thyem I could kill deathclaws at level 1 with a 10 mm pistol. Console gamers, amirite?

@Visible Earth

I am guilty of this as well but on Fallout 2 during that trial quest where you have to blow up a wall. It took me awhile to realize that the darn thing was ticking in my pockets the entire time when I kept activating it and blowing up after moving away a fair distance.

In Fallout New Vegas I placed a mine into Easy Pete pants to show him how well i understood how explosives worked, then i proceeded to eat his corpse obviously.

It was my first new vegas playthrough, for some reason I thought it was a good ide heading north through the quarry, even after the npc warned me it was full of creatures. At the time I had never seen a deathclaw and didn't know what to expect, so I wait for night time and start to sneak towards Vegas while I was shitting my pants in pure fear of what would ambush me, then I look behind me and see this lizard thing running at mach 5 towards my direction, before I could even let out a scream he just one-shot'd me.

The stupidest thing I had ever done in any Fallout was heading to the Master's base at level one in Fallout 1. Fucking terrible idea, especially when I didn't know how to play the game.