Season 4, Episode 11 - The Justice League Recombination
Aired December 16, 2010
With New Year's Eve approaching, the guys invite Zack and Penny to join their Justice League group at the comic book store's annual costume party. Their hopes of finally winning the contest this year are dashed when Penny pulls out.
Season 5, Episode 13 - The Recombination Hypothesis
Aired January 19, 2012
Leonard and Penny might start things up again when Leonard offers Penny a spur of the moment invitation to dinner, on the 100th episode of The Big Bang Theory.