Revealing mistake: On many occasions throughout the film, Keira Knightley's real, short hair can be seen under her wig.
Revealing mistake: Throughout the dance scene between Darcy and Elizabeth, a small earpiece can be seen in Elizabeth's ear. I would assume this is to help with the dance steps. This can also be seen during the dance with Mr. Collins, however it is not as noticeable. (00:39:00)
Answer: In previews to American audiences there were lots of comments that it was not a good ending (not romantic enough) so they filmed an ending with Darcy and Lizzie kissing to make it more romantic. The ending in other countries matches the original ending of the book.