When Phoebe is returning the dog, a woman with a blue shirt in the background walks into her house twice.
When Ross and Chandler go outside to fight the bullies, the same male extra in a blue shirt and a woman in a brown plaid jacket exit the coffee shop together twice.
At the end when Monica dances on the diner bar to the YMCA, fake Elvis was on her right but in the next shot fake Lucille Ball is there instead.
Monica says she can't work at the diner after having been sous chef at Café des Artistes. But it was shown in Season 1 that she actually worked at the restaurant Iridium.
Right before the dog appears on the taxi's window, Rachel is facing Joey but when the camera angle changes to include the dog in the shot, Rachel is facing the front of the taxi without enough time for her to have turned her head. Moreover, even if she had enough time to turn around, the dog must have already appeared on the window and she should have seen it before Joey did.
This is more of a mistake of the restaurant where Monica works. At the end of this episode, they play the song "YMCA," which came out in 1978. Being that the restaurant has a 50's theme, this song would not make sense.
When Ross, Chandler, and the two bullies run after the men that steal their watches and money, an extra bumps into a fire hydrant which moves.
When Phoebe's dad's dog is hanging on the window of the cab, you can see the top of a crew members head who is crouched next to the cab to hold the dog up.
At the end, when Chandler falls down behind the couch, you can see the mat he falls onto poking out from behind the couch.
When Monica says, 'Time is money, my friend' and then hangs up the phone, you can see the microphone for a second.