Leonard: It wasn't until his twenty-first birthday that 1955 Biff placed his first bet.
Sheldon: Wow wow wow. Is 'placed' right?
Leonard: What do you mean?
Sheldon: Is 'placed' the right tense for something that would have happened in the future of a past that was affected by something from the future?
Leonard: 'Had will have placed'?
Sheldon: That's my boy.