I forgot one....
Did a pre-flight check on a Slingsby which had just landed. Everything looked ok, so I started up and taxied towards the runway. When turning onto the runway to line up, there was a noticeable jolt. It didn't feel right, so I taxied back to the maintenance hangar. The previous pilot had done a very heavy landing, without telling anyone, and seriously damaged the undercarriage.
It would have failed on the next landing.
My check had been too cursory, assuming that there couldn't be that much wrong with an aeroplane which had just landed.
Moral: be careful, and never trust the last pilot who flew your aeroplane.