Rachel Green [Friends] & Ben Wyatt had a relationship at some point
In Season 7, episode 9 of Friends, Rachel is saying that she doesn't sleep with guys on the first date, when Monica brings up a few names, including Ben Wyatt.
Both shows were NBC, so it wouldn't be too difficult to imagine they're in the same universe. After doing the math, at the time of Season 7 of Friends (2000), Rachel is 29 years old. At the same time, Ben would be 26, slightly younger but still within the range of a relationship. During this time in Friends, Rachel is dating her assistant, Tag, who's butt she often mentions she finds 'cute'. Leslie often says the exact same thing about Ben's butt during their relationship.
So in conclusion, I think at some point after Ben left Partridge, Minnesota in shame after his failure as mayor, he went to NYC for a brief time, met Rachel, and either had a one night stand, or a relationship.