The One With Phoebe's Husband - S2-E4
Character mistake: When everyone hears that Chandler has a third nipple, even Ross is surprised. He's know Chandler for many years and would certainly know this fact.
MovieFan612

The One With Phoebe's Husband - S2-E4
Visible crew/equipment: After the opening credits, just as Joey says that it's unbelievable Phoebe can be married, a camera and yellow electrical cord can be seen at the left side of the screen, and a bit of it is still visible in following shots when Monica mentions the cheeseburger.
Super Grover

The One With Phoebe's Husband - S2-E4
Continuity mistake: In the scene where Rachel is telling Ross it is sexy for a man to wait before having sex, the cups behind Ross change color: red to green to red and back again. (00:08:10)
Pam-I-am