Colin Firth As The Voice Of Paddington In "Paddington", Replaced By Ben Whishaw
Colin Firth was the original voice of the adorable bear. Firth willingly left the movie as it became clear to all parties involved that his voice was not the right one for the part. Without Ben Whishaw's unique demeanor, which he imparts to the two flawless movies, it is difficult to conceive the marmalade master.
Also, how would anyone be able to tell when some background character in GoT was replaced? There were a ton of them, and they all looked the same.