Deliberate mistake: For dramatic and cinematic purposes, the floor of the court where the last scene takes place is wet, as opposed to the circle where Tony dies.
Deliberate mistake: Gonzo and Rizzo are selling apples in the beginning, including the Red Delicious cultivar. That wasn't invented until the 1880s and the story is set sometime in the first half of the 1800s. Most likely intentional as they are the narrators.
Deliberate mistake: Rocks on the water suddenly appear when the rabbit shows up, which allow for Alice to chase him.
Deliberate mistake: Several times during the movie the narrator explains what happens while the book flips its pages. During this, every page is identical to the last one so the animation has simply been copied/reused to save time and money.
Deliberate mistake: In "Do You Hear the People Sing?", Grantaire can be seen hijacking the funeral carriage, but when the insurrection starts, he is at the tavern with some girl, and Marius says, "Grantaire, get off your arse, it's begun!" and acts like he wasn't just there. George Blagden, who portrayed Grantaire, didn't want to be left out of that great set-piece, so he was allowed to participate, and the director must have thought no-one would notice him. (01:36:55 - 01:38:45)
Deliberate mistake: During the song Adelaide's Lament, Adelaide goes to her medicine cabinet, opens it, and there is a cut to a shot from "inside" the cabinet as she surveys all the medicine she takes to treat her "cold." From the inside, the cabinet is almost twice as wide as it is when it shows her open and later close it from the outside.