Boys' Night - S2-E18
Other mistake: In the opening scene, the Dunphys are playing ball in their backyard when Claire knocks it into their crotchety old neighbour's yard. They all head in, and as Phil explains to Luke that it's because it's their only ball, another ball is in the grass visible over Luke's shoulder.
Phil on Wire - S3-E3
Other mistake: When Cameron follows Mitchell into the sea, Cameron's shirt is visibly wet even though he is in a very shallow area of water. After cutting back to Cameron, the wet mark on his shirt has gotten bigger and traveled further up his shirt even though there are no significantly large waves that could have caused him to become so wet.
Other mistake: When Claire is sitting in first class, the woman she was sitting next to finished her drink while on her angry phone call and asks for more, but about a second later she is drinking it again and it is no longer empty.
Answer: He has seen himself dancing, and is mortified at how bad he looks.