Forgetting the death grenade of a psycho is up there on my list of most annoying mistakes along with walking off or backing off a cliff…I’m always blown away by 1 life runs because I marvel at how they can go so long without walking off a cliff
I only do that in the Peak now. Backing up and falling off in Decimation Destination. The worst part of that fall is I often land on a ledge, so I’m still alive but can’t get back up into the arena, so my run ends with a quit, not a death.
I could probably grenade jump or rocket jump back up, but I’m so bad at those maneuvers that quitting is the same thing.
–RoA
Executing into a EXP or Hot Loader (with no Grog).
Being a noob ninja, these are all things I’ve done a number of times.
I am exactly the same. When I get it right, however, it is so rewarding.
Some overlap with the answers above here, for me it’s highly character/skill dependent, like Paul said.
Zer0: Run into own kunai.
Krieg: Wait too long for RtB.
Maya: Phaselock the wrong thing.
Axton: When I first picked up Onslaught I ran off cliffs a lot, not any more. Now? Run into my own Babymaker reloads.
Sal: Reloading.
Gaige: Mistiming Discord.
Sigh, yes. With Krieg I’m less concerned about getting fried, but executing into them or killing them at point blank range… I always realize “oh yeah that’s a Hot Loader I shouldn’t be this close” right as it dies and explodes. 
Oh, and another, since I just did it again;
Forgetting you have a longbow delivery grenade mod on.
Toss out a grenade into a small mob thinking to reduce 'em, and watching in amazement as your 'nade flies towards the horizon and vanishes in a tiny puff hurting no-one but your pride.
–RoA
And another. I haven’t done this in about a month, but dammit just did.
Hitting your sprint button on a controller while driving a vehicle and ending up switching seats instead of speeding up.
Yes, you should remember that you have to hit another button.
But dang it your go fast control should not change when you transition from foot travel to vehicle travel, there is no reason for this. Would be easy to make switching seats the car boost button and then us busy vault hunters would not have to remember, just go on instinct. And no, you can’t customize it away because the two actions are linked.
Really a pain when you hop into a car because you’re in trouble and need to get away fast, and you end up switching seats and getting pummeled instead.
–RoA
I’m so used to TPS mechanics and every time I play BL2 now, I’m trying to use my OZ Kit after jump. I mean after every jump, it’s so annoying and kind of funny at the same time.
Hitting the wrong button to Dashboard when I’m being picky about which quest item I receive. I’ve screwed up and taken a few non elemental Sandhawks due to this. It pisses me off every time but I can only blame myself … so frustrating when it happens.
So long as you don’t trigger the autosave you can still dashboard out of the game and try again…
I can name a few:
Hoarder Salvador: Not using my Fastballs to get NKLO up and rolling. Assault Rifles almost need NKLO to dent let alone wreck healthbars of mobs.
Maliwan Gaige: Uh, I’m pretty sure I didn’t intend to Discord there.
Hyperion Maya: Wait… I’m not supposed to use the Heart Breaker as it’s not as quick as Wreck. FFFF–
Deputy/Umpire Sal: Remember kids, Torgue Ravager on the MAIN hand on the first loadout. Jakobs shotty on the OFF hand of the other one.
Dahl Axton: No. NOOOOO. ALWAYS ADS on this brand. ALWAYS. You don’t use the full potential when hip-firing!
You can use a grenade to jump start your NKLO? Well I learned something new!
My annoying mistake is fighting my way up to the tower in Sawtooth only to realize i forgot to bring proper corrosive weapons to take down those pesky buzzards.
Throwing fastballs too near to a enemy ( and therefore to my face ) and going down with him
Shooting at an enemy with the Peek Opener, behind cover, only to go into FFYL because of the grenades… Then dying because I’m too far away from, and behind cover, to kill anything… So frustrating!
I’ve always been a bit irritated with grenade (and rocket) self-damage. Our digistruct weapons are smart enough to not harm our teammates or ourselves with bullets, why aren’t our grenades designed with that self safeguard?
–RoA
Don’t reflected bullets do damage? I mean from the loader with the helicopter arms…lol… Or when you’re trying to take out Hyperious’ minions and their shields deflect bullets back to you… and kill you. If you drop an Anti Infection next to yourself, you’ll go into FFYL. That’s what I mean with the Peak Opener… The main projectiles make it beyond my cover, but the grenades it uses as the secondary attack apparatus, manage to stay behind and knock me into FFYL.
It depends, and you can somewhat understand the mechanic. Ricocheting bullets from our weapons, like a Madhaus for example or Gaiges close enough , do not damage the player or teammates. Bullets that ricochet from an enemy change allegiance (to stretch an analogy) from your to theirs and then do damage, and that makes some sense.
But a grenade I toss that never switches control to an enemy should not affect me. Take a transfusion grenade, as an example, it’s seeking child particles always target enemies, not players.
–RoA
Yeah, I see what you mean. But, if the seeking grenades explode near you, even if it’s on an enemy, and they have an elemental effect, the effect does do damage to you. Grenades have always done damage to yourself if tossed close enough, hence why grenade jumping works. Rockets, it seems, are coded to work the same way as grenades and is why rocket jumping works, as well. It’s the ricocheting bullets you fire that doesn’t do damage to you or teammates that is the mystery.
You’re right. Mini-IFF transponders built into the bullets?
–RoA