Dance Dance Revelation - S2-E10
Revealing mistake: When Lily hits Mitchell in the face, her 'arm' is covered by a pyjama-like material, but the next shot shows her wearing a sleeveless dress, showing her 'arm' was really a padded stick.
Revealing mistake: While Phil and Claire are racing each other to Manny's birthday dinner, each car goes around corners, but the steering wheels are never turned.
Answer: He has seen himself dancing, and is mortified at how bad he looks.