In the pre-credit scene, Joey is asking who wants tickets for his play later in the day. After asking Monica and then saying, "Two it is," Joey turns to Ross and says, "Ross, how about you?" Ross doesn't have anything in his hand but in a close-up he is holding a potato chip.
When Tommy starts shouting at the duck it changes its position.
After the play, Joey asks Kate how she is doing, and she says "fabulous". She brings a cup to her mouth, and then does it again from another direction.
When Gunther brings Chandler the cup of coffee from the woman at the bar, Chandler does not take it by the handle; however the shot quickly switches to him holding it by the handle.
It is highly unlikely a drama critic can have their review of a live stage production printed in a newspaper within hours because the review has go through an editing process.
When Joey is backstage saying goodbye to Kate, someone yells, "Joey, on stage!" No one would yell backstage at a live production.
When the group arrives at the theater, they are clearly approaching the orchestra from the far left aisle. However, when Tommy is screaming at the couple for being in the wrong seats, he states that the seat number is 14 and that they're surrounded by even numbers. Most playhouses number the left side odd and the right side even, with center orchestra seats numbered consecutively.
While waiting on the phone, Phoebe switched Monica's and Rachel's rooms. In the morning, Monica came out of her original room, where she should have come out of Rachel's room. Response: They could have decided to sleep in their original rooms or they could have moved their items back.
Any time Monica and Rachel's phone base is visible while Phoebe is shown waiting on hold, it is clear that there are no cords so the phone is not connected to anything.
Rachel waited two hours to use the apartment phone, but there is an available phone in Joey and Chandler's apartment across the hall.