Deeply puzzling -- how to explain the anomalous pieces?

Christmas is jigsaw time, and this year has deepened the mystery of missing and extra pieces. This closeup shows a piece of paper we placed beneath one hole in an effort to conceal the gap. Some of the problem puzzles are recycled ones I got from the White Rock Library to feed seasonal jigsaw habit. But some are brand new.

Missing pieces are easily explained. They slip between sofa cushions, or pets chew or move them. It's a bit harder to explain how wrong pieces get in a puzzle box. I imagined a crew of library volunteers around a table, counting pieces to make sure the donated puzzles were complete. The table was crowded and two people sitting side by side couldn't keep their piles separate. Unwittingly, they exchanged a couple of pieces without altering the count.

Three extra pieces in each puzzle above. Both also have gaps, yet none of the spare pieces fit. I guess life is like that, sometimes.