The One with the Evil Orthodontist (S01E20)

Barry (now engaged to Rachel's ex-maid-of-honor) is still pursuing Rachel. She goes to his office to put an end to it and winds up having sex with him. Mindy asks Rachel to be her maid-of-honor, then confesses that she was with Barry while Rachel was engaged to him. Rachel tells Mindy she just slept with Barry again, and they both go to break it off, but Mindy winds up remaining engaged to him. (Turner Broadcasting System)