When the first victim, Kathy Jordan, is in the steel room, you see Donny pour the accelerant on her torso and she is visibly wet. The next scene shows her dry again on the torso.
The hallway outside the steel paneled room is brightly lit but when Donny is shown, from Kathy Jordan's point of view, to enter the steel room, the hallway outside is dark.
Mother can be seen breathing when Donny finds out she is dead.
Toward the end of the first burning scene, it is obvious that the fire effects are a simple overlay.
During Donny's dream, the camera focuses on a large hole in the sand on the desolate beach. However, three people can bee seen wandering on the beach far away from the hole at the top of the screen.
At the beginning of the movie, when Ben get's burned, it's obvious that in the shots before he is just in normal clothes, but when he's on fire, he's wearing a fire suit under the clothes.
There is no doorknob on the inside of the steel paneled room. As such, Donny would not have been able to get out of the steel room once the door latched closed.
When Donny is in the steel room, he obviously is able to exit the room after the death of his first victim. But when Suzanne and Patty are in the steel room, neither of them can escape despite the one woman not being chained to the floor and ceiling.