Champions (great game, miss it, lots!)
I had a gadgeteer-type character who was susceptible to sonic damage. Our group was a really great bunch of guys except for one - Dave Wall. Few of us tolerated Dave, and the rest just plain old didn't like him. His characters were always min/maxed for whatever character type he wanted to play. If it was a brick, he maxed strength, defense and speed, sacrificing intelligence, skills, etc.
On that particular night, we had managed to fall into the villain's trap, and were all imprisoned by our own weaknesses. I was strapped to a chair with headphones, with the threat that as soon as I attempted to move, speak or through anything other than effort of mind, my character's brain would be jellied by the sonic attack. They conferred for quite some time before devising a domino plan where one of us would free me, and then I would free the next, who would free the next, etc.
Our weathermaster-type was Dave, who was encaged in a neutral humidity/ionazation and whatever else the GM could use that prevented him from using his powers - in the cage. It was decided that he would try to use his powers beyond his imprisonment. The GM stated that it would take a perfect roll of "3" due to not being able to determine all the necessary factors (such as being able to feel the humidity, etc.)
In a great roll he was form precipitation/condensation around my headphones and short them out, getting the necessary "3" to be successful. The moment I became free I was able to free our telepath from his trap, by repairing the headphones and focused the beam on his restraints, who then went on to free our brick.
Our brick went to free Dave by, basically, destroying his cage. Unfortunately for Dave (and to the extreme but well-hidden delight of the rest of us) he rolled the proverbial "18" - a dead failure. Dave's cage imploded, taking Dave with it.
The look of sheer dejection on the guy's face and the unwillingness of the GM to allow Dave any "divine intervention" was awesome.
For my worst moment, I recall Champions, as well. I was playing an acrobat/martial artist-type. One of the bad guys was holding a gun to an npc's head. Thinking that I should have no problem foiling this, I let my ego take over my brain, and proceeded to get the npc killed when I tried to wrest the gun from the bad guy. It sucked, and I truly felt like crap in real life for having gotten the red-shirt killed. However, I gotta say that it was in character, as I had taken glory-hound as a limitation.
To this day, I carry a picture of the little npc's he left behind.