While your friend is sleeping deeply, get out a non-permanent marker and write the word "Gotcha", or something to that effect, backwards on his or her forehead.
This is a gotcha for many novices because they miscalculate the amount of border they'll need by not allowing for waste at the corners.
One big gotcha in design is achieving the right scale for a room.
Gotcha, that was what we in the know call a 'Rickroll'!"
He owed Fred a "gotcha" after the hard time he'd given the old man about his electronic sales adventures.
Fred gave him a big gotcha smile.