Nightbreed (1990)

Revealing mistake: During the big final fight at Midian, at one point, one of the men with a flamethrower accidentally stumbles backwards into a patch of burning shrubs/plants, which sets him on fire and eventually causes his flamethrower tank to explode. As he accidentally stumbles back into the shrubs, watch closely- one of the plants starts to fall over, and is attached to a small square board hidden in the dirt below that becomes visible, revealing that the plants were pieces of set-dressing.

Boone: That's true.
Peloquin: Everything is true. God's an Astronaut. Oz is Over the Rainbow, and Midian is where the monsters live... And you came to die.

Trivia: The post-production process of the film was very dramatic. The studios became worried that the subject matter was too strange for audiences, and in an attempt to make the film faster and simpler to understand, they demanded that nearly an hour of material be removed from the film. Director Clive Barker also maintains that the film wasn't marketed properly. A Director's Cut of the film (containing much of the cut footage) was announced more than 20 years after the film's original release-date.