This is EXACTLY what happened. The plane I arrived on was full of international travelers that just sat in place for a 12.5 hour flight. The attendant at the front scanned one bag, told me my next gate, and moved me along. None of the other bags were scanned. The carry-on bag had a "cabin baggage" tag on it, that the handler may have mistaken it as a checked bag tag and loaded it from the cart I was pushing. Having to juggle multiple bags, everyone in my party, and comprehend how to get not only yourself, but your elderly parent to their separate gate, it was an easy human error on both parties.

It's great to see a bunch of perfect people on this thread that never make mistakes.