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.
Answer: He has seen himself dancing, and is mortified at how bad he looks.