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.).
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.
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.
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)