Cliff needs a date to the Postman\'s Ball, and accidentally ends up with dates with both Carla and Diane.
This is one of my personal favorite episodes thanks in part to the classic scene where Cliff brings his hippie co-worker (who talks to himself) to meet Carla, and the painful awkwardness when Cliff tries to woo Diane.
This is a great character study as much like Louie DePalma had his loveable qualities, Cliff does too, only for him to ruin any sympathy with his own self-interests.
And speaking of the Taxi references, could it be possible that Lucas (the hippie mailman) may have been a small tribute to Rev. Jim?