in a busy supermarket like Sainsburys on a saturday, you finish shopping and join a horribly long queue.
Suddenly you remember something which happens to be in the next aisle along, so you leave your trolley in the queue and dash to get said item.
When you come back you expect your trolley to be in the same place and not to have been moved to the side and an old bloke with a basket in front of you starting to unload his basket.
Shopping etiquette says you wait your turn right even if the owner of the trolley in front is not there for a couple of minutes.