Attended a training course in the late 90s for which I booked hotels for myself and a colleague. Paid via credit card on one of the old carbon paper machines, and claimed the expenses from my employer as normal. However, the charge from the hotel never appeared on my credit card bill. Called the hotel and they said they had a record of me paying so nothing to worry about. Kept that ~£500 aside for 3 years in case it ever showed up on my statement, but it never did.