...when you don't let the brownies cool completely before you turn the pan over.
I was baking a big batch of brownies to send to my son - who is on deployment in the middle of the Indian Ocean. (It was also so I could get the brownie mix out of the kitchen!) I let them cool for a while, loosened them all around the edges, and tried to turn them over on a board.
I was able to salvage 2 dozen pieces to send to Kenny, and Don is happy that he gets the rest!