This looks totally staged, but there's a video on YouTube of a guy pranking his girlfriend by telling her he CHEATED. But it backfires, because she tells him SHE cheated too.
Then after he flips out, she reveals she's just JOKING, and that she saw him set up the camera. Also, she's disappointed because she thought he was going to PROPOSE.
(Search for "Anniversary Prank Backfires." He tells her at :44, and she flips it on him at 1:14. Then she reveals she's just joking at 2:04.)