Dickens originally considered calling Tiny Tim "Little Larry", "Small Sam", or "Puny Pete".
In the manuscript version of the story, Tiny Tim's fate after Scrooge reformed wasn't revealed, with his survival presumably added when Dickens went to the publishers'.
Word of Dante: Several details of the story have been used in so many stage and screen adaptations that it's surprising to learn that they weren't in Dickens' original.
In adaptations Belle is usually seen dancing at Fezziwig's ball, sometimes being his daughter or niece. In the book she's first seen when she breaks up with Scrooge, and no relation to Fezziwig is given.
The reason for Scrooge's hostility toward his nephew is never clearly spelled out, though most adaptations assume it was because his mother Fan died giving birth to him. Likewise, the reason that Scrooge's father is cold to him is never spelled out, but is often given a similar Freudian Excuse.
Mrs. Dilber is often made the unnamed housekeeper who stole the curtains, sheets, and shirt off Scrooge's corpse, while in the book she was the laundress who stole his sugar tongs, boots, and spoons.
Word of God: At the beginning Charles Dickens speaks directly to the reader to impress upon them that Jacob Marley was dead to begin with. He explains this one fact is absolutely crucial to the story, and therefore warrants extensive Word of God confirmation, from death certificate to door-nail.