Standing outside the club among the three guys who see Lily and Robin flash their breasts is Samm Levine, who was cast alongside Jason Segel in the 1999-2000 Freaks and Geeks (1999). When Marshall (Segel) walks by, Levine says he looks familiar and tries guessing who it is.
Samm Levine (who appears in this episode) originally tried out for the part of Barney but stepped aside as he thought he wasn't right for the part.