Continuity mistake: When Super arrives at the Daily Planet to defy the 3 villains, the outside shot shows three piles of books lined along the window. From the inside, all of the books are piled to the right. Also, the number of trophies on the bookshelf on the right, and the type and number of books changes between shots.
Continuity mistake: Inside the Fortress of Solitude, when Non jumps to fight Superman, before he flies, Ursa and Lois are looking at Superman. A mere frame later, when Non starts to fly, both girls are in a different position fighting against each other. It's a really awkward and sudden change.