Continuity mistake: The scene in which the whole family were mud-fighting. Just after the camera man says "Excuse me, are we still doing this" look at Manny's shirt. It just has a small mud stain on the pocket, but as the scene continues the stain looks different with no reason and there is suddenly mud on Manny's face too.
Continuity mistake: After the photographer asks Claire if they are still taking the picture Claire says no and puts mud on Gloria's belly. You see that its a round stain. In the shot after that the stain on Gloria's dress has become just a smear. This changes again in the shot after that. (00:20:05 - 00:20:40)
Answer: He has seen himself dancing, and is mortified at how bad he looks.