Revealing mistake: At the beginning when the ruler is making his speech, you can see his modern day fillings in his upper molars.
Revealing mistake: When Ash shoots Cheryl through the door in the head, you can clearly see the hose (not an artery) that's squirting out the blood. (01:16:14)
Revealing mistake: When Wally is firing the laser at random from the small spaceship, we see Fidget running down some stone stairs, and there are violent laser impacts directly behind him that could only have hit where they did if they had passed right though his body. (01:41:55)
Revealing mistake: When the two station reporters are investigating Eddie's room, as the female bends down to the table, she is scared by a dog. In one shot you can see a black wire on the dog. (00:14:55)
Revealing mistake: Morgana has smoke coming out of her mouth as she recites "The Charm of Making". If you look closely the smoke is being expelled from a tube next to her left cheek that at times is not even close to her mouth. (02:06:00)
Revealing mistake: In the scene right before the dragon picks up Ulrich, you can see the dragon pass through Galen and Valerian. (01:41:45)
Revealing mistake: The first time David werewolfizes, we're treated to the complete process... hair developing, bones painfully lengthening and twisting... and the torso of the actor very visibly halfway in the floor while all this is going on to the full-body werewolf prosthesis. Oh for computer graphics at the time to erase that detail of the process.