Episode 80 of "Felicity"
s04e18 Time Will Tell

Felicity becomes distant and withdrawn after Elena dies in a car accident. She pulls away from Ben. On the day that she receives an invitation to Noel and Zoe's wedding, she catches Ben kissing Claire, the couple's friend and study partner. She breaks up with him and calls Meghan, who urges her to come to New York immediately. Ben later arrives for the wedding and apologizes, but admits that he was seeing Claire for a month. Felicity tells Meghan that she wishes she had gotten together with Noel. Meghan casts a spell that sends Felicity back in time to the night that she slept with Noel. This time, she quickly confesses to Ben. He wants to forgive her and move on, but she turns him down because she is hurt by what he did to her in the future. Felicity approaches Noel and says that she wants a relationship. He initially becomes confused and pushes her away, much to her surprise. However, he later says that he wants to be with her, and is just afraid that she will reject him again. Felicity becomes unnerved by all of the events that change because of her decision to change the past (or present, or whatever it is). Ben falls into a state of despair and doubts his decision to take pre-med courses. Elena and Tracy argue about Felicity's indiscretion and call off their wedding a few days early. Because they don't make it to the altar, Sean and Meghan cannot get married in their place. Felicity asks Meghan to undo the spell, but Meghan proclaims that Felicity has lost her mind.