After Chandler carries the ugly punch bowl into the bedroom, it magically reappears on the dining table as Phoebe and Monica talk.
After Chandler goes to see if Monica has a bobby pin, Joey uses air quotes when sarcastically saying "Monica". However, in season nine's The One Where No One Proposes (2002), Joey tells Ross he doesn't know what air quotes mean.
When Phoebe is massaging Monica the second time, the sheet she has over her is purple, then when the camera goes back to Monica on the massage table the sheet is pale green.
When Joey's feeling the kick of Rachel's baby, he is not without underpants like he said so.
Chandler opens Monica's secret closet by pulling the pin from the hinges. This is not possible to do because of how the pin portion of the hinge sits in front of part attached to the door. As soon as he pulls the pin, the door falls slightly to the right, and you can only see the door half of the hinge, meaning the hinge wasn't actually attached when the pin was pulled.
Chandler states he has never seen the inside of the locked closet by the bathroom door in Monica's apartment, but in "Friends: The One with the Routine (#10.6)" (1999), you can see Rachel open the closet door while he, Rachel, and Phoebe are searching for Monica's hidden Christmas presents. So he has seen the inside.
When Ross enters Rachel's apartment, a boom mic can be seen above him for a couple of seconds.
In this episode, Monica tells Phoebe that she won't let her massage her because she would feel awkward, and then they talk as if Phoebe has never massaged her. However, in an earlier episode, Phoebe tells them she has only massaged two people that year, in which Monica then states that the two were her and Ross.
It's rather unusual that Phoebe knocks the Monica's door in the very first scene. Friends never knock when getting inside Monica's or Joey's apartment.