My first flight after earning my heli rating was to fly to an airport picnic to give a few rides in an R22. I bought fuel, and did all the normal preflight checks, including checking both fuel caps. These had a black mark on the round periphery that lined up with another mark on the fuselage when fully tightened. That makes it easy to see if it's properly attached.
Upon landing, and the crowd looking over the machine, someone asked what the hole was on the top. Hole? WHAT HOLE? The left cap was missing, and my knees weakened when I realized it could have taken out the TR. I still don't understand how it happened with the alignment marks and I was certain I had fully tightened it. With great luck, the tractor used for cutting the airport grass used the same very cap, which was loaned to me for the return flight.
What amazes me is these caps aren't on a tether chain so they can't depart the aircraft, or be set aside and forgotten.