Continuity mistake: When Ross is telling Monica and Phoebe that he found his "sound" in college, Monica spits out her orange juice and places the glass on the coffee table. The shot changes to Phoebe and you can see Monica putting the glass down again.
Continuity mistake: Chandler is in Rachel's apartment after he has just left Monica; he says to Rachel, "You look great", then Rachel puts her hair behind her ears. The camera cuts to Chandler, then straight back to Rachel, and her hair is not behind her ears.
Continuity mistake: There is a long shot of Rachel and another person sitting on the sofa and Rachel's wine glass is almost empty. She then goes over to the the counter to fill it up. However, before she starts to fill it, a significant quantity of wine can be seen in the glass.
Factual error: This mistake is from series 6 onwards, with Ross' new apartment: There is an impossibility with Ross' kitchen, but not all of the time. It depends on the layout of the hallway outside his apartment. If the hallway continues along for a bit before going left or right, then the kitchen should stick out. However, when the hallway is a dead end, the kitchen fits ok.
Continuity mistake: Monica and Chandler are in their apartment talking about Monica's worries that Chandler would dump her if she put on weight. When we first see Monica, the bite mark in her apple is quite small, but a few seconds later, it is suddenly much bigger without her eating any more of it.
Continuity mistake: When the group is sitting around the table in Monica's apartment playing poker, Monica is making something to eat. When she puts it on the table and sits back, we can see a pile of plates on the table on her right side. But in the next shot, the plates are gone.
Continuity mistake: Near the start, Chandler is sitting in the coffee house in the big chair. When he's talking to Joey about money, his feet change positions every two seconds. On the floor, crossed, resting on the table, the list goes on and on.