Setting details that come back to bite you. The players never, ever forget those ones. They'll forget the name of their own king, but not that they can take on a dragon without fear as long as they wear violet because dragon's eyes don't work in that part of the spectrum (a stupid fact you came up with six years ago to explain something completely different about elves that the players pestered you about).

Transcript

Obi-Wan: I wrench the droid through the air so the blaster bolt hits it instead of me.
GM: You don't want to use your laser sword to deflect it?
Obi-Wan: At a -8 penalty for unsteady footing? No way.
[SFX]: Kchink! {blaster bolt hits the droid; Obi-Wan falls}
GM: See now, this is what you should have been worried about when you leapt through the window. You're now falling thousands of metres to certain death.
Anakin: Master!
Obi-Wan: It's perfectly safe. Remember the field generators?
GM: Uh...
Obi-Wan: That's why Coruscant has no railings. There was never any danger. One does not forget important setting details such as that.
GM: It's not the fall that will kill you. It's the landing in the lava pit.