The only time I've seen this cause a problem is when people leave their belongings on the tables leading up to the X-ray in order to dart around a slow person at the head of the line ... in which case, they're counting on the folks behind them to put their items into the X-ray! (Presumably without stealing anything ... ) When this happens, I will tell people, politely but firmly, that they need to stay with their items until they're all inside the X-ray.

OTOH, if someone alarms on the first pass and goes back to divest their pocket change, etc., I'll usually wave the next few folks through, as long as they've put their items into the X-ray.

We really DO want to get you through the checkpoint as quickly as possible, folks!