When the moths shatter Glen's window, he panics and runs out of his room to get Al, leaving his door open. In the next scene Al approaches Glen's room and the door is completely closed.
When Glen sees the first Geode that's split apart, he holds one half in his left hand and the other remains on the ground. When he stands, he's holding onto both halves (one in each hand).
When Al and Glen are in the downstairs closet, A. drops a handful of shotgun shells. They scatter in front of the inside of the door, along with the demons outside. When the scene pans up and over them, the shells are gone.
When Glen breaks the geode open with the hammer, it is clearly shown cracking dead straight down the center. Yet, moments later he touches it and it falls in half, the half that falls over is much smaller than the half that remains upright. Had the geode really cracked down the middle, the two halves would have been equal in size.
When the Lee sisters leaving the house, they have garlic around their necks. When they get outside, the garlic is gone.
Neither geodes nor thundereggs form in lightning strikes. Both are of volcanic origin. Technically, a geode is not the same as a thunderegg, which is a filled nodule. The actual residue of molten ground in lightning strikes is called a fulgurite, which is lengthy, with branches. It can indeed be very valuable.
When Glen is descending the crumbling staircase there is a brief shot of his legs searching for a foothold. The legs are clearly those of a female stunt double.
(at around 40 mins) When Al, Glen, and Terry are in the backyard setting up the rocket, Al crouches down with a pair of pliers to tighten a bolt. She's not tightening anything, just moving the pliers back and forth in mid-air.
Obviously a doll of dead Angus, because when he falls out of the car. He's too light.
Obviously fake wound when Glen pulls splinter out of his wounded hand.
The word "business" is misspelled in the note to Terry from his father.