After Courtney Cox married David Arquette, they added
'Arquette' to the end of the entire cast and crew's names in the credits to
honor the marriage.
In the two episodes "The One That Could Have Been" (Part 1 & 2) the opening
credits have been changed: you see the alter-egos of the cast members in
the "alternate reality" (Monica still being fat, Joey as famous actor
etc.).
At the end of the 2nd part of the episode "The One With Two Parts" the
epilogue is totally syncronized in Spanish, a reference to Marcel turning
the TV's language into Spanish at the first part of the episode.
When the "The One That Could Have Been" was divided into two parts for
syndication, a sequence was added that had the ending credits superimposed.
It shows fat Monica dancing to some music while eating some food. The
original hourlong airing of "The One That Could Have Been" did not have the
credits at the end of the first half hour and this scene was not involved.
The credit clips are changed regularly, about twice a season. In TOW London
the clips are only those that occur in that episode (Such as Chandler and
Monica waking up in bed together, and a pregnant Phoebe eating cereal on
her belly)