If you use a paper punch to make holes in paper for classifying in ring binders, have you ever wondered where those little disks of confetti appear from after you've put the punch away? Sometimes the piece of paper sticks to the punch instead of falling off inside the collector, and on its way back up as the punch clears the hole, it jams against the sides of the newly formed hole. If you put the paper in the folder immediately, you force the "blank" out of its hole, but if you don't, it will just sit there for a while, until it eventually falls out.

With sheet metal, which is thicker than paper, the punch-out (that's the word my book uses) may not re-enter the hole fully. The hole may be "filled", but there will be an indentation on one side of the sheet and a projection (aspérité) on the other. If the punchout twists sideways as it is wedged (like a butterfly valve wedged in the open position) there will be an "aspérité" on both sides.