Revealing mistake: After Mr. Heckles has been taken away, in the last wideshot while everyone's still standing in the hallway, when this is viewed on the DVD (Warner Bros. S2 set) we can see a long black line on the floor (as noted in another mistake). The problem is that when this is viewed online on Netflix, even though we can see the very same area of floor, that black line has been blurred out. Weird that it wasn't edited on the DVD, and yet it was for the version available on Netflix.
Visible crew/equipment: After Mr. Heckles is taken away, when it cuts to Phoebe walking out of the kitchen, the reflection of studio lighting array can be seen on the TV screen. Also later in Heckles' apartment, studio lighting's reflected on his TV screen.
Visible crew/equipment: After Monica yells "Stop with the broom, we're not making noise," when both Monica and Rachel stomp their feet the studio lighting can be seen at the top of the screen, over Ross (this doesn't refer to Monica's star light fixture). And when the others join in stomping their feet, we can also see the boom mic dip into view.
Continuity mistake: After Monica breaks the seashell lamp, when Phoebe says that she wants to believe Monica did it accidentally her right is arm up and points to Monica, but in the next shot it's Phoebe's left arm that's up while still pointing at Monica.