Now I am going to really blow your mind.

Heinlein was in the middle of writing Stranger in a Strange Land when some political events transpired which made him stop working on that novel and write Starship Troopers. He finished Starship Troopers and then went back and finished Stranger in a Strange Land.

To this day, nobody except the people closest to him at the time (who have all died) knew where he stopped and restarted work on Stranger in a Strange Land.