Deliberate mistake: The shot of the mothership landing bay and the arrival of Charles' ship is stock footage that has been used several times before. There is also a continuity error here - the view from the landing bay shows darkness outside, but the next shot shows broad daylight. (00:06:20)
Revealing mistake: The two Elizabeths are both played by the same actress, except in the shots where both are seen together. Continuity mistakes in the second Elizabeth's death scene reveal this, such as the second Elizabeth's head position changing, and the first Elizabeth's jacket being open/closed between shots. (00:42:30)
Continuity mistake: Mike Donovan (riding a horse) is chased by a Visitor shuttle. It is obvious that there were recycled some shots from "The Final Battle" (Ep. 1). In "The Champion", Donovan does not have a laser pistol. However, there are some shots in which he has that pistol: obviously, those shots are taken from TFB.
Revealing mistake: When the two wildcats throw hand grenades at the Visitors, the grenades are obviously fake because the explosions do not occur at the same place as where the grenades land. The detonations have obviously been located in a safe place away from the actors. (00:11:50)