Continuity mistake: Arthur ties a rope around the battlements and descends the tower's west wall. Tony then waves a torch to signal the soldiers to attack. He's standing at the same spot Arthur left from (the rope is still tied there), but in the long shot, his torch is waving from the tower's south wall instead of the west one.
Factual error: The aliens show Doug and Tony the planet they plan to invade - Earth, of course. But the special effects department forgot to give poor old Earth any clouds. (There are clouds over more than 70% of our planet at any given time.) When they go down to the surface, though, there are plenty of clouds in the sky.
Continuity mistake: In the saloon, the table at the lower right of the shot has nothing on it. After a brief close-up of the bartender, we cut back to the master shot - and even though no one's gone near it, the previously empty table now has a candle in a tin holder sitting on it.
Continuity mistake: Doug and Tony are being interrogated by the Mars Mission flight crew. In two-shot, they're standing shoulder-to-shoulder with each other, no more than a few inches apart. When the shot cuts to a wider angle, Tony is suddenly several feet away.
Continuity mistake: Jiggs goes back with a bag of grenades and a machine gun. He fires the gun, but when he is jumped on by the Trojan, the bag is gone. An object which may be a bag is lying on the ground, but is not the one he brought. No further mention is made of the grenades.