Chess Opening Theory/1. e4/1...e6/2. d4/2...d5/3. e5/3...c5/4. c3/4...Nc6/5. Nf3/5...Qb6/6. Bd3/6...cxd4/7. cxd4/7...Nxd4

7...Nxd4?
This seemingly obvious move is a mistake. Black just lost his knight to a discovered attack threat.