It’s been held for additional checks and wasn’t loaded on time.

Some employee, usually a novice, put the luggage on the wrong trolley, and it flew somewhere else.

An airline employee put the wrong sticker on it by mistake.

The passenger hadn’t removed old stickers from the luggage, which lead to confusion.

The suitcase got stuck on the belt or went in the wrong direction, which happens during connecting flights.

The suitcase fell off the trolley on the way to the plane and no one noticed. This, however, is rare.

The luggage recognition system made an error or the label got lost. That’s why employees advise leaving your name and contact information where it can be seen clearly on the outside or at least inside the luggage.