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.
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.
Disneyland - S3-E22
Revealing mistake: The episode was filmed with real Disneyland visitors. One can constantly see them staring at the camera and/or Disneyland cast members standing with their backs to the actors, monitoring the crowd. This is especially noticeable when the family first arrives and when Mitchell and Cameron talk about Lily's leash: behind the actors, there are long white rope lines supervised by park staff to keep visitors at a distance. Several people can be seen filming the actors or taking pictures with their phones.
Revealing mistake: In the scene where Claire catches Alex and Skyler about to shave each other's hair, Claire grabs the shaver and waves it around, and the shaver gets caught in Skyler's hair. The color of the hair caught in the shaver (black) is different from Skyler's hair color (very dark brown). It is quite close, but still significantly different. (00:11:50)
Answer: He has seen himself dancing, and is mortified at how bad he looks.