1.91. A man wakes up one night to get some water. He turns off the light and goes back to bed. The next morning he looks out the window, screams, and kills himself. (CR; KK wording; originally from How Come -- Again?)
1.91 answer: The man is a lighthouse keeper. He turns off the light in the lighthouse and during the night a ship crashes on the rocks. Seeing this the next morning, the man realizes what he's done and commits suicide.
1.91a. Variant, similar to #1.70: The light goes out and a man dies.
1.91a answer: The lighthouse keeper uses his job as an alibi while he's elsewhere committing a crime, but the light goes out and a ship crashes, thereby disproving the alibi. The lighthouse keeper kills himself when he realizes his alibi is no good. (From Eric Wang)
1.91a variant answer: A man commits a heinous crime, claiming as his alibi that he was onboard a certain ship. When he learns that (due to a lighthouse failure) the ship was wrecked without reaching port safely, he realizes that his alibi is disproven and commits suicide to avoid being sent to prison. (From Eric Wang)