Continuity mistake: Season 03 episode 11, "The Demon Reborn": When Lois recovers in her apartment from the knock-out gas and starts swinging, Batman pushes her against her nightstand. But once she calms down and the camera makes a wide shot from the ceiling of the room, Lois and Batman suddenly crouch quite a distance away from the nightstand.
Deliberate mistake: Season 2, episode 6 "Identity Crisis": When Bizarro starts decaying into the parody form of Superman, his costume goes with it; the blue fades, and even the 'S' insignia changes. Strange that superhero garments change right alongside their wearers when they suffer from an unstable genetic effect. Were the clothes cloned right onto his body, instead of sewn?
Factual error: Season 2 episode 10 "Double Dose": When Superman snatches Parasite's escape boat and hauls it back to Stryker's, Parasite attempts to blast him with a shotgun found aboard. But somehow the animators appeared unable to decide whether the gun should be a double-barrel or a pump-action type; and building a double-barrel pump gun would be impractical due to additional weight and bulk.
Superman: A game has rules. Your stunts are just random idiocy.
Mxyzptlk: Okay, I'll give you a rule. You make me say, spell or otherwise reveal my name backwards, and I'll split until our dimensions come into alignment again in about, oh, three months, say give or take.
Superman: I can't even say your name forward. How am I supposed to say it backward?
Mxyzptlk: No, dope! You don't have to say it backward. You have to get ME to say it.
Superman: Say what?
Mxyzptlk: Kltpzyxm! Gosh, you're thick! Now, for the last time - ah, nuts. [Disappears.].