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: The beginning to series 7 takes place right after series 6 ended, basically Chandler has only just proposed to Monica. Yet Monica's hair is visibly different; at the end of series 6 she had no fringe and a center parting, now she has a fringe. Also Chandler, is visibly thinner, with a gaunt face due to losing 20 lbs in between the two series. Rachel also has had a hair cut, her long extensions have been removed and she is back to her usual hair length.