In the episode where Liz lost her diary, Max gets a job in the U.F.O. museum. They go in the archives of this museum and they're talking. There is a shelf next to Liz, where there are some green books (they look like folders). One of them is different: clear and smaller. The camera frames Liz and the small folder is there; the camera frames Max, then Liz again, and the folder has disappeared; then it re-appears and disappears again :)