Washington had a game-changing play called back in the second quarter against the Carolina Panthers.
Chris Culliver was called for his helmet-to-helmet hit on Greg Olsen, negating what was a huge play. The hit didn't look particularly violent, but consulting the NFL rulebook shows why this one was called back.
"It is an illegal launch if a player (1) leaves both feet prior to contact to spring forward and upward into his opponent, and (2) uses any part of his helmet (including the top/crown and forehead/"hairline" parts) to initiate forcible contact against any part of his opponent's body."
It may not have been the most egregious helmet-to-helmet hit, but the fact that he left his feet is probably what doomed him.
Olsen was arguably a defenseless receiver, too. There will be plenty of discussion about whether or not the right call was made here, but you can at least see why they made it.