One afternoon, a long time ago, I took a passenger for a short flight in a school/club C152. The aircraft had been used all day for students, and I was assured it had been filled up after every one-hour lesson. Being short, I need a ladder to visually check the fuel tanks on high wing aircraft, and I was only going for about half an hour, so I believed it. The gauges were registering full - not that I took any notice of that anyway.
When we got back, the person who filled up said there was less than half an hour's worth of fuel remaining!!!!!!!!!!
Needless to say, I now get a ladder, every time.