When Betty is snipping some flowers in the wraparound story, before the final story, the buds are all closed. In the nest shot they're all open.
The position of the groceries in Betty's bag change between shots in the first segment of the wraparound story.
After Drogan dies, he drops his bottle of pills. In the next shot the spilled pills and the bottle are differently positioned.
Position of the keys after Betty drops them in the final segment of the wraparound story.
The amount of whiskey in Halston's glass as he notices the gash on his shoulder.
In preparing to cook Timmy, Betty remarks that she could never do long division and asks aloud how much is 75 divided by 12. When Timmy answers and asks why, she indicates that she is trying to determine his cooking time. But if she is, she must multiply 75 by 12. (The answer is 900, or 15 hours.)
Drogan blinks after he dies.
When the camera is simulating the cat in first person, you can see shadows of strings that are on the camera.
After Drogan is dead in his wheelchair, he blinks just before the camera cuts away.
When the kid is going through the Tales From the Darkside book in his cell you can clearly see that the pages in the book are blank.
Obvious fake cat as it attacks Carolyn's face.
When the credits are being introduced, it's noted that the list is in order of appearance. However, Matthew Lawrence actually appears before Christian Slater and other names are obviously not in proper order.
In the wraparound story, Timmy and the witch both make clear that she's made him "eat cookies all day long" so that she can fatten him up while he's in captivity. He's visibly not fat.
Kneepads on Amanda as she rolls downstairs.