These have bothered me every time I see these episodes of Friends.

First, in the episode with Chandler's Work Laugh, watch the end where Ross is telling Chandler that he had a thing with Janice. They're in Chandlery & Joey's apartment, and Ross is near the counter and stools. Chandler is sitting in his chair in the living room. During this conversation, many times Chandler has his legs crossed different ways every time the camera cuts back to him. I realize it's just a mistake due to the different takes, but it's still pretty noticeable.

Also, watch the Rachel's New Dress episode. When she and Joshua are at his parents' house and they walk in the living room to discover her in her "dress", she has her legs crossed one way. The camera cuts away, and then back, and they're not crossed that way. Then cut away and back, and they are. This continues a little bit. After a commercial break, you return to this scene and her legs are again placed differently. I can attribute this to her being shocked and nervous by having Joshua's parents see her like this, but it's again a pretty obvious mistake on Jennifer's part.