Revealing mistake: When Reggie and Tim the sheriff's son are in the funeral home and get confronted by the two black women, the silver ball comes to attack and kills one of the women. Tim saves the day with a revolver, but when he spins the cylinder before firing, you can see all of the cartridges in the gun have already been fired: dimples in the centre of each casing.
Continuity mistake: The beginning of Phantasm III starts with a flashback from Phantasm II. The car being driven in the flashback is a 1971 Plymouth Hemi 'Cuda, with four headlights, two distinctly separate hood scoops, and a hardtop. In Phantasm III, the car is a 1970 Hemi 'Cuda, with two headlights, a single Shaker hood scoop with dual openings, a "Hemi" stripe, and a convertible. It's supposed to be the same car, and in the flashback sequence it's easy to see the differences.