Joe invents a time machine. He travels back in time and meets Emily, and they have a child, Bill. Bill grows up, meets Carol, and has a child, Joe. Joe grows up and invents a time machine, and so on.

Joe and Bill are each the other’s father and son, and each man is his own grandfather.

From Dave Morice’s Alphabet Avenue, 1997. See “Proof That a Man Can Be His Own Grandfather” and Oedipus Wrecked. Robert Heinlein’s 1959 story “‘–All You Zombies–‘” is even more confusing.