Other mistake: In the episode with Ross's wedding to Emily, Joey is talking on a cell phone with Phoebe as he walks down the aisle. Joey holds the phone out to Ross saying 'it's Phoebe', and one can clearly see that the phone's screen is completely blank. The phone isn't even turned on.
Continuity mistake: When Phoebe is talking to her stalker for the first time, at one point, he tips the flowers to the side when Phoebe says, 'Well, this is awkward,' and he puts his left hand on the end of the stems. In the next shot he is holding them with both hands.
Plot hole: Chandler and Ross are surprised to notice that Joey has very small feet. It's well beyond suspension of disbelief to think that at the least Chandler (who, apart from being his best friend, has been his roommate for 8 years) has never noticed the size of Joey's feet before.
Continuity mistake: When Monica is standing in front of Phoebe, asking if the smell from Joey's is bothering her, the shots of Monica from the front show her arms held out in front of her, but when the shot changes her arms are crossed. This happens twice.
Other mistake: There are two mistakes in the scene where Phoebe spots Joey's picture in the laundromat. For one thing, when Phoebe says "Oh, here it is", she is looking in the opposite direction of the picture. Also, when she says this, her mouth doesn't move at all.
Continuity mistake: In Joanna's office, Chandler and Rachel are pulling his pants from one another while he is cuffed to the filing cabinet. Right before Rachel says "What if I clean your bathroom for a month?" she throws the pants onto a chair, but then when Chandler later says "Freedom. I want my freedom. Why won't you hear me?" he opens the door to yell to Sophie and they are both holding the pants again.