During the opening scene in the coffee house when Monica says the other girls surprised her by throwing her a bachelorette party, Rachel's arms switch suddenly from being by her sides to fidgeting with the string on her top.
Monica comes into the coffee shop to tell Chandler he is having a bachelor party. When she sits on the sofa arm, her purse is in her hands/lap. A couple of shots later the purse is around her shoulder.
In the opening scene, Phoebe's purple coffee cup keeps changing orientation. First the handle is pointing camera left, then camera right, then at her, and so on.
In the closing scene, Rachel puts the phone on the counter as she leaves, with the earpiece and mouthpiece facing her, away from the door. In the next shot, the phone is facing the audience.
In the opening scene, Mona leaves and Joey is thought to say "so you and LAURA..." , (instead of Mona). However, he just says the name oddly. He still says 'Mona'.
Monica states that Chandler should think of the stripper not so much as a bachelor party, but as "a 2-month anniversary present". Seeing as how their wedding was in May, however (clearly stated multiple times in seasons 7 and 8), she should have said: "a 5-month anniversary present".
Joey uses the "finger quotes" to tell Chandler that Ross was "conveniently" too busy to attend Chandler's bachelor party. However, in The One Where Emma Cries, Joey tells Ross he doesn't know what the "finger quotes" mean.