Harry Potter and the Prisoner of Azkaban - Hermione's Rock
This slip-up is when Hermione, Ron, and Harry are in Hagrid's hut for the first time, a jug is smashed and then they realize that Dumbledore is coming with the executioner, so they leave. But first, Hermione looks at the rock. It's clearly a rock with a little fossil of a snail's shell somewhere on the upper corner. However, later, when Harry and Hermione go back in time, it shows Hermione throwing a large snail shell. Not a rock, but a shell. It's trivial, I know, but it bothered me.