In all the games I've ever GM'd, which were not many, I have to confess, I've implemented a "fate point" or "drama point" system, inspired by Warhammer roleplay, which so far has been very much liked by the players. The idea is that every character has a certain number of said points and can spend them for various effects in-game. Usually, we use the following rules: one point spent means something likely but not guaranteed happens (a guard at the gates turns out to be a lazy bum and half-asses the search of your cart), two points spent mean something unlikely happens (an otherwise mortal blow just clips your shoulder and drops you to the ground dazed instead) and three points spent mean something very improbable but still possible happens (the enemy crew turns out to be dead drunk due to the captain's birthday party the night before and takes appropriately large stat penalties). Points are awarded by the consesus of players and GM for good roleplaying and impressive in-character deeds.
One of my friends used... in GURPS, I think it was, a plot point to make it the commader of a station's birthday so there were very few guards.
They were SUPPOSED to capture the station. They nuked it in the end.
This reminds me of a Marvel Super Heroes game I was in, where we were fighting Quicksilver and a bunch of random mooks on top of a skyscraper. We ended up with Quicksilver dead, the random mooks dead (keep in mind that killing anyone in MSH was a big no-no and you lost all Karma points), the top several floors of the building destroyed in a nuclear explosion, and most of the team in an ICU.
Spectacular fails included:
-A huge bruiser character getting knocked off the building. A flying power armor character zoomed down to catch him, but failed his roll and winded up getting slammed into the pavement and knocked unconcsious. It did manage to save the bruiser though (also knocked unconscious).
-My character (whose only powers were Flight and Density Manipulation) using his powers to basically become a flying brick (that....was actually his superhero name - The Flying Brick) and hitting Quicksilver square on, no mean feat considering he was zipping all over the place. Unfortunately, the knockback sent him flying over the edge to his death. Oops.
-Another character getting a critical shot on what we thought to be a mind-controlled Luke Cage. Unfortunately, it was an android imposter, and the critical shot caused it to self-destruct (and it was nuclear-powered). Cue large boom.
We were supposed to have another chapter that night, but with everybody in intensive care and wanted for murder.....