Revealing mistake: When Louis is trying to get Gage off him near the end of the movie, as he's struggling in a few shots you can plainly see it's not really Gage but a dummy made to look like him. Looks like a doll. (01:33:35)
Revealing mistake: When we first see Max, his paws are leaning on a balustrade and become transparent. Though it may seem like it's an optical effect because they're in the shadows, the next angle shows them fully lit and opaque.
Revealing mistake: When Nick is being sucked out of the hole/cave, you can see the wires visible from his waist. (01:20:35)
Revealing mistake: In one shot when Alice is in the shower, as she turns for the soap you can tell she is wearing brown underwear. (00:03:30)
Revealing mistake: During the Hy Brasil sinking sequence you can see a horizontal line in the water in the background, this is the edge of the water tank they used to film this sequence. It's also visible in several shots from the longship.
Revealing mistake: When Ray, Winston and Egon are underground looking for the river of ooze, there is a scene where they break down a small wall of rocks. Ray then sits on one of the rocks and it bends under him - it's rubber or foam. (01:01:50)
Revealing mistake: In the scene at the graveyard, after the Warlock assembles the Grand Grimoire and commands it to say God's true name, the fake headstone behind him starts to fall over. (01:33:02)
Suggested correction: Other headstones can be seen falling before this scene.
Revealing mistake: The time on the scoreboard clock when Ray hears the "go the distance" message changes from 10:30 to 10:32 in a matter of seconds.