When the gang looks out the window at the person looking at them through a telescope, Phoebe's position changes between shots - she's first between Ross and Rachel, and then between Chandler and Joey.
Rachel's missing bra in Barry's examining room disappears quite a few times between shots. One moment it's hanging on the dental equipment, the next it's not, then it reappears, and so forth.
Monica and Rachel discover a guy across the street with a telescope spying on them. Later, Joey says that the same guy has been spying on him, too. However, Joey's apartment is across the hall and his windows open into an alley. It's not possible for the guy with the telescope to spy on Joey as well.
Joey said the guy was looking into their apartment, not their room. Their living room window is facing out into the street the same as Monica and Rachel's window.
Joey said the guy was looking into their apartment, not their room. Their living room window is facing out into the street the same as Monica and Rachel's window.
When Rachel is looking for her missing bra in Barry's examining room, one can see through her shirt that she is already wearing a bra.