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.
Gotcha, that was what we in the know call a 'Rickroll'!"
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.
One big gotcha in design is achieving the right scale for a room.
Fred gave him a big gotcha smile.