In the Flashback, both Ross and Chandler say they each had sex in high school. But in Season 1, Ross admits that Carol was his first and they met in college. In Season 4, it was revealed that Chandler was 19 when he touched a woman's breast for the first time.
When Rachel and Monica are dancing in the flashback, they are both holding cups. It is obvious that nothing is in the cups, but Rachel drinks from hers as Monica fetches the pizza. There isn't enough time for her to fill her cup even when she is not in the shot.