When Phoebe and Rachel are in the coffee house talking to Erin, Phoebe's slice of cake changes from one with white icing, then back to no white icing.
At least twice, Ross appears to say "li-berry" instead of "li-brary". This is a common turn of phrase in the NY/NJ/CT area, however.
In the beginning of the episode, Janice comments on Monica's engagement ring during their conversation in the kitchen at the restraunt where Monica works. However, Chefs usually don't wear rings while cooking in case they are accidentally dropped in the food.
When Monica walks in to tell Chandler about seeing Janice, a mic is visible directly above her.
Ross tells the grad student that he skipped 4th grade. Ross is only one year ahead of Monica in school. (He was a freshman in college when she and Rachel were seniors in high school. "The One Where Ross Meets Elizabeth's Dad") If he had skipped 4th grade, he would be the same age as Monica and not a year older.