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?
Man to Man, Beast to Beast - S1-E6
Deliberate mistake: When Bruce Banner is crawling away from the town that Hulk attacked, he passes out in the snow and is rescued by Sasquatch/Walter Langkowski. When he awakens in Langkowski's lab, he is wearing his usual outfit. Since he was only wearing tattered pants when he passed out, that means his clothes were given to him by Langkowski, which is extremely unlikely since the two hadn't seen each other in a long time and Langkowski didn't even know Bruce was looking for him.