I wasn't charged for a flight about 20 years ago. It was for a frequent flyer award. I redeemed two awards, one transatlantic and another within Europe. The agent got confused and deducted only the transatlantic part. It was ticketed separately but maybe was counted as a stopover. Anyway, I've been keeping running tabs and there were a few flights that I could never get the airline to give me credit so the overall tab is about even.