Ok, time to fess up. This is the place to admit those times where you just stop thinking and do something completely moronic.

My defining one would have to be when I remote detonated the breaching charge I'd placed at the start of the run without factoring in the structural damage several minutes of sustained suppression fire had done to the building. Not moronic in and of itself... except that the character had a degree in structural engineering and could have simply made a knowledge check to spot the problem before hitting detonate.
End result: I dropped a building on myself.

your turn.

i bought a 200k car for a rigger char.
bought extras worth 600k for the car.
forgot to buy anti-theft-systems.
parked it out on the street.
i got to drive it once.
i got to see it drive around once or twice in the distance -.-

Stormed a building to rescue someone from what we thought were her kidnappers. Turns out that she had gone into hiding and the other people in the building were the security detail she'd hired to protect her. Oops.

Some players in a game I ran once decided to kill the bored old security guard on the front desk of a low-security apartment building they had to enter, even though I described him as reading "Not Paying Attention Magazine". They shot him gang-land execution style, in the centre of the forehead, then when they realised that would make it obvious that professionals had been in the building, they roughed up his corpse to "make it look like a robbery". Needless to say, the boys in blue were less than convinced. Idiots.

I am sorry... this just seems like a dick move from the GM to me.
Even if you parked it in the barrens someone would still need to either hack the cars node or physically hardwire the car.
For hacking, you would need 9 hits, rating 3 node + 6 for admin, which is not exactly easy for gangers. Unless your gangers are rocking decent skills and great programs... And yes, you would need admin access. User access may allow you to turn on the radio or open the door but to actually drive it? The car I drive around in now would have better security otherwise...
For hotwiring, it would be a major rework of the wires and not just crossing the starter wires coming from the ignition key since even modern cars can turn off the engine when that nifty RFID is not in the car.

My team was moving through an alleyway, when we were ambushed from above. My adept threw a grenade up onto the roof to clear the ambushers. Glitched, and the grenade fell back down into the alleyway in the middle of my team.

The confined space alleyway.

Sounds a lot like the British SF Character in Twilight 2000 who tossed a grenade, poorly, and it hit the jamb of the door and bounced back up under his legs. Shortest played character I have ever seen. 1st Encounter, 1st action of the game. *shakes head*

Shortest due to how long he lived or his size after having his legs blown out from under him?

Nice...

I can't recall when I've done something colossally stupid but a while back, the rest of the group was adamant about storming a gang's HQ to steal back a drone (or at least its parts as they had found some records that the gang had dismantled it). I refused to be a part of their harebrained scheme and sat out the game. Without any real planning, they drove a delivery van we had stolen in a previous run and charged right into the compound. It did not end well.

We had to get into a building, full of gangers, with our extractee in the basement. Since we knew that a knock-down-drag-out was likely to get our mark shot (the longer it went on the more chances the gangers would have to geek him), I decided the logical thing was to burst through the wall in a modified SUV. That's when I learned that builders in the barrens don't care much for "Building Codes" and brought the building down on top of us (in our ride). We had stick-n-shock and gas grenades ready, since we wanted to keep the body count down. Fat lot of good that did.

Got the mark out safe though.

@Mikado:
SR3.
No such thing as hacking.
It's croaking open the door then hotwire it nd off you go.
And as i said, i was dumb enough to park a several hundred k worth car out in the open for everybody to see in the barrens and left it there over night, expecting it to still be there because:"it's player-property, the whole character will be basically useless without it, the GM won't do that."

I stand corrected. However, hotwiring still takes time and since you did not give a timeframe to work with (over-night) I was just basing it on the parked it on the side of the road for a meet (or something that would be an hour or so of time) it would be a dick thing, regardless of the location.
having it sit overnight... then yes, I would not expect it to be there...

Even my GMs(yes, plural) would not be that mean to me . . well, at least usually . . but that was just too stupid to not punish, and i accept that.

How long he lived. (IMG:style_emoticons/default/smile.gif)

Gone in 60 Seconds... (IMG:style_emoticons/default/smile.gif)

Playing a combat hacker, I was running for my life because the run had gone to hell. Got to the roof and looked around for options. GM said it was a 6 meter gap between buildings. I jumped, then realized I'd forgotten to buy any Athletics skills for the character.

Landed in the alleyway ten stories below with a satisfying crunch and proceeded to make a new character.

Ooh, completely dicked up?

Well, there was this one game--wasn't Shadowrun--where I pretty much fell asleep in the middle of the game; some...thing was in the water being a large pain the ass, and we were to take care of the problem (murder whatever the fuck it was) and I awoke to "Ok Draco, what do you do? There's a writing green mass rising out of the lake."

Me: "Shoot the FUCK out of it!"

It was my partner.

Good thing I rolled poorly and missed.

Recon mission. Was spotted by one guard. Yes, one. Was taken out easily. Instead of falling back the general idea became, "Well, we already killed one. Might as well take the rest so they can't track us down for revenge." A recon should NOT have a double digit body count.

Oh, another time, this time Shadowrun.

We were hired to steal a crate from some warehouse. Almost botched the job, a LS heli on our tail as we're swerving in and out of traffic to avoid it's HMG.

Crate is taking a few shots, leaking cryogentic fluids, and the containment device goes into an emergency reactivation procedure (IIRC, he was the only one with the Drive skill besides myself...it was either that, or the highest default on the dice pool).

I, the rigger, hand the wheel to the shaman and get in the back to over-ride the awaking process (which failed, ultimately).

That turned out to be a bad idea, as he took the nearest exit off the freeway to go "underground."

Into, and then through, the Renraku Mall Complex.

I think we killed or injured a hundred people.

In a twist of fate, we ended up doing an extraction mission a week later at Renraku.

Shaman summoned a watcher spirit and told it to haunt the memorial that had been constructed, so that they couldn't take down the (obviously temporary) structure. It would moan and so on when they tried.

This was during our old SR3 campaign years back (we've since moved on to SR 4) where we had this mission to take out a single person in a small hidden base in the mountains (snow and etc) there were armored jeeps and tanks parked outside the base, and most of them were manned.

Now you see, due to the craggy landscape and such there were large badly drawn rock formations about, and the GM had run out of minis representing some of the vehicles, so he had drawn some onto the mat (which I had conveniently forgot, and none of my teamies had reminded me about this)

And that's how I almost died for jumping in front of a 'guitar shaped rock' .

I know it's that other game with double D's, but my dwarf warden was pretty low on the whole critical thinking/charismatic behavior bit. I was getting tired of the game since the DM was throwing level appropriate encounters at us without handing out the nifty upgrades to anyone but his wife (needing 18+ to hit something EVERY fight gets demoralizing after a while). We needed to convince someone to give us some information, and the ranger and charisma thief were new to the game and failing hard. So, my solution to pretty much any and every social encounter when we weren't getting our way? Punch whoever we were talking to in the junk.

Doing this, I snapped one man out of some sort of mind control spell, and took down a bandit leader before his minions attacked. (IMG:style_emoticons/default/biggrin.gif)

Playing a phys adept Hobgoblin brawler. Who carries grenades as his ranged.

Got kidnapped in a "restaurant" that was actually a bank vault in disguise (wtf?), and got out of it 40 minutes later to find ourselves out to sea, carried by freightchoppers. And we're some ways out. (GM's justification for this shit was so thin it was transparent, but GM=GodMode, so)
We decided to climb the cable up to a chopper for hijinks and giggles.

GM has an NPC run electricity down the metal cables. We pointed out to him that, without a ground, 400 ft above sealevel, we would not get shocked. His bad.

That being unsuccessful, the chopper guys cut the attaching cables free.

Now, my partners had no-where near my climb ability (and they did not have gecko gloves, I did), so I was the only one to make it up by the time the cables disappeared. So I punched a hole in the side (the SIDE, not the bottom) of the chopper, hanging on the bottom like a spider-monkey to avoid being in a line of fire. Nevertheless, several people inside started making "holy shit" noises. So I decide that a grenade might solve things nicely.

Not being a COMPLETE idiot, I chose a DMSO splash grenade with Narcojet. I figured I'd escape the blast.


So I pull the pin, lob the grenade through the hole...
Which promptly flies out the open door I never checked for, on the other side of the chopper.

After pissing off Kyle Morgan after Mercurial, Bull the Ork Decker was at dinner. He sees an item on the electronic menu called "Death Au Gratin". Thinking it was just an amusingly named item, he presses the button to order that.

I woke up two days later in the Hospital with my first cyberarm after the menu exploded and took my arm with it.

Bull

So, you fell for the old 'press X to die' trick? Can't blame you, it always looks like a joke until too late.
Backtrace whoever thinks they're so clever for adding the button and give them a couple of buttons of their own... you know, stuff like 'Press X to die in acid trap, press Y to die by drones wielding chainsaws, press Z to die by chickens...'
Make sure that the chicken death is suitably spectacular.

Or have you already done this?