Three of the main characters were the same actor, and yet there are shots with all of them in the same scene. When I saw this as a young adult I didn't even notice that Mike Myers was playing three roles and was genuinely dumbfounded years later when I found out.
Orphan Black was great. The most insane part was how you could tell which character was pretending to be another character even though they look the same. Tatiana Maslany did fantastic work on that show.