Whether this story is true or not (or just the names have been changed to protect the innocent) this is why I carry two copies of boarding passes AND my receipt.

I keep one in my personal carry on in addition to the one I show/handover to the employees. Should I lose one set it is unlikely that I would lose the other;

it becomes especially useful when making sure I get credited properly for the flights flown. It would have been useful for showing that she paid for F and somehow was stuck in Y.