^Or TNG: "Conundrum," sort of. The other characters don't remember MacDuff, since they don't remember anything, but he's just suddenly there as if he's always been. So it's another case where we know the character's out of place but the cast doesn't.
I remember when that episode first aired, I was royally
confused. Who was this MacDuff character? I kept thinking back to previous episodes trying to figure out if I had seen him before. It didn't dawn on me until about halfway through the episode that he might be key in what was going on.