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.
Continuity mistake: Right after Clark reveals himself to Lois in Niagara Falls hotel room, by his hand falling into the fire, he backs up to where the bed and lava lamp like column is behind him. In that shot, there isn't much of the "lava" in the column. The scene cuts to a shot of Lois and then back to Clark and suddenly there is a lot of "lava" in the column. (00:53:05)