Continuity mistake: Mitch and Cam are eating breakfast and Cam only fills Mitch's wine glass. As the camera view goes back and forth a few times it keeps switching to show Cam's glass full then empty and so on. (00:08:30 - 00:09:30)
Continuity mistake: In episode 8-20: All Things Being Equal, while Lily is going to a feminist protest with Claire, Gloria, Haley, and Alex, Alex said something like 'The women burnt their bras', and Lily says 'Not me, I just got my first bra'. But in this episode Lily steals a bra because she's embarrassed to ask Mitchell and Cam to buy one for her.
Answer: He has seen himself dancing, and is mortified at how bad he looks.