In one of the scenes in “Memento”, there is a flashback of Sammy sitting in a mental hospital. Someone passes in front of him and the scene changes back to the current time. Just before the shot changes, there is a flash of Lenny in Sammy’s place. I noticed this the second time I saw the movie and guessed that in some way, Sammy was actually Lenny. This page explains the plot in more detail: