Jennifer Aniston's actual last name is Anastassakis but her family changed it to Aniston when they moved to the United States. If you watch TOW Chandler and Monica's Wedding, Jennifer's actual last name is used for the Greek couple having their wedding in the episode.
All the friends have a lot of expensive things in their apartments, but you have only seen them lock their doors twice.
The spare room (Rachel's) in Monica's apartment has a wall right opposite the door, which serves absolutely no purpose except to block the view into the room from the living room. The bedroom set isn't always built, just when needed, so having that wall makes life much easier. It's completely illogical though, as it's too thin to be a supporting wall, and where it is makes getting into the room a pain.
Matthew Perry and Bruce Willis starred together in the movie The Whole Nine Yards. While making the movie, Matthew made a bet with Bruce that the movie would open in the top position at the box office. If it did this (which it consequently did) Bruce had to star in an episode of Friends for no fee.