Several scenes supposedly occurring in the same location were obviously shot on different sets. This is because the shots involving American actors were shot in the USA, whereas the shots involving Spanish actors were shot in Spain.
(at around 34 mins) The jar with one slug collected from the garden was given to the scientist and was after shown to contain multiple slugs when removing one for dissection.
(at around 9 mins) When Mike Brady and the sheriff are driving down the road in the police car, the camera on the driver's side clearly shows that Brady's window is rolled up, but in the next shot over on the passenger's (Brady's) side, he throws his cigarettes out the window.
(at around 25 mins) When the man grabs a slug from the garden that had bitten him and puts it into the jar, his hand is free of blood despite having had been bitten and covered in blood in the previous scene.
(at around 51 mins) Mike Brady says a line before he's supposed to. Towards the end of the movie, he barges into the commissioner's office and tells the secretary, "Will you tell Mr. Phillips that Mike Brady is here to see him?" The secretary replies, "Hey, his office is closed." Brady says, "Yeah, watch me!" but only then the secretary says, "You can't go in there!"
Normal slugs don't have teeth, but these are mutant slugs.
(at around 1h 20 mins) During the sewer explosions one of the manhole covers can be seen beside the opening without anyone around to have opened it.
Spanish actors.
When the whino enters the front door of Ron Bells house in the beginning, the shadow of the boom operator can be seen on the adjacent wall.