As with many of the entries on this list, I find myself with a little bit of an interesting relationship to the prompts, because I don't have a lot of fun stories that focus on me (as a GM first and player second, I'm often a little too experienced and gunshy to fall into many of the wackier hijinks).
(questions and image from autocratik.blogspot.com)
However, in my first game I was running there was an interesting plot arc where I'd put a scenario into play where the players were supposed to break into an arcology and steal from it.
For the reference, this was the Renraku Arcology in Seattle, just around the time of Third Edition Shadowrun.
The original plan was to have players encounter Very Bad Things inside the arcology, as that was what the plot arc focused on, and while I wasn't following any published adventures I was following the canon, if nothing else.
The players were sort of scattered, but there was the troll I've previously mentioned and a handful of other over-armed and over-augmented characters who were really good at the corporate part and not so good at the espionage part of their job description. Overpaid and underworked, I believe the expression goes.
In any case, they show up to a mission and are achieving an admirable degree of stealth, for an 8-man group of general miscreants, when a couple of the party members (who are split off from the rest of the group) decide to go Splinter Cell and shoot their way through to their objective. In their defense, they didn't have stealth skills worth mentioning or disguises that would hold up (and, to quote an oblivious CLUE Files player, "I have no social skills" was the motto of the day).
Since time was critical, they figured that they'd get in and out before anyone noticed the piles of corpses left around the secure areas.
Cameras, drones, and other considerations notwithstanding, they didn't have silenced weapons (the troll was part of the other group).
As a GM, I decide to be merciful and let the group get a relatively small consequence for their actions, in the form of a couple investigating guards. They're high-tier, so not easily bypassed, but not quite the elites that you would otherwise find in such situations. A shootout ensues and the players are outmatched, rolling really poorly and generally not doing hot.
I'd had any hopes of the campaign proceeding as planned more or less dashed at this point, and the other players found that there was a new security alert making it difficult to proceed, and got the hint (in no small part due to having out-of-character knowledge about how badly outmatched their friends were; they did have radios so I hand-waved this as hearing the screaming and shooting over the radio).
One of the guards throws a grenade, hopefully forcing the players to beat a hasty retreat ahead of the numerous incoming emergency personnel (who will soon have bigger fish to fry, but that's not a concern for the players), and maybe try to regroup.
Lo and behold, one of the players jumps onto the grenade.
Blasts in Shadowrun are interesting, because they're basically semi-worthless half the time and really good half the time. Against an armored tough opponent, like both of the players in the hallway at the time, the grenade had no real threat unless the space was tight. The space they were in wasn't small enough to cause this (after all, the guards would've been in the splash zone if it was), but if anything happened, the damage would go up exponentially.
As it stood, however, the worst either of the PCs had to fear from the grenade was a scratch, since they were so far (miraculously) unscathed.
So one of them jumps onto the grenade, invoking the "enclosed blast" rule and generally getting as positively wrecked as possible. Barely alive, he gets rescued by his buddy who gets the hint that it might be time to leave.
On the upside, they weren't trapped inside the arcology. However, the hopes of the System Shock inspired follow-up sessions and the poor PC's intestines were ruptured by a lack of willingness to re-evaluate the current lay of the land.
Of course, this failure wasn't really my own, so if you want my own failure, I can point to a time when I had conversion values hit me hard.
In the same campaign, shortly after the grenade incident, the players were looking for some petty crime to build up their cash reserves, and I had them ambush a truck carrying some gold, because I was not really paying enough attention and they rolled really well on their "let's find a job" tests.
Long story short, they manage to stop the truck and steal its contents, at which point they ask "so how much gold was in here?"
Panicking and not really sure what the value of gold was, I say "About a hundred pounds" or something like that.
Long story short, the PCs got a gear upgrade, and they definitely built their cash reserves.