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.

Trivia: In the original cut of the film, the character Narcisse (the man with the partially skinned head who Boone first meets in the hospital, then travels to Midian with after being resurrected) was killed by Decker during the final battle sequence. However, his character proved to be popular during early test-screenings, so his death was cut from the film, and director Clive Barker shot a few new scenes with the character to expand his role slightly.