In the beginning of the movie, Dr. Beck appears older than Mrs. Slater when she loses her baby. Later on, Mrs. Slater is much older, but the doctor doesn't seem to be much, if any older than he was when shown in the opening scene.
(at around 41 mins) Diane locks the kitchen door and remains next to it. In the next shot, she's leaning against a counter. In addition, later, Katie runs out of the kitchen, without anyone unlocking the door.
(at around 58 mins) When the girls crash the dumpster into the police car, it is stuck against the front of the car. However, when the police officer tells them to move it back, there is suddenly enough room between the dumpster and the car for them to push it back in the other direction.
(at around 1h 13 mins) When Vicki is killed, her demise comes as Mrs. Slater's cane comes down upon her left eye socket, spurting blood as it impales. When Dr. Beck and Kathryn discover Vicki and Liz's bodies in the open grave at Park View Cemetery, Vicki only possesses her chest and back wounds, no eye damage.
When Kathryn flees into the second upstairs bathroom stall she discovers Jeanie's decapitated head in the toilet bowl and the blood from her murder all upon the tiles surrounding the bowl. Jeanie was murdered and decapitated in the third upstairs bathroom stall with her head come down to the toilet bowl within that stall.
(around 1 hour 26 mins) The first three shots of Eric wearing the clown costume in the attic with Katherine, it clearly shows Eric's actor without his wig. The next shot Eric's actor wears his wig underneath the clown hat.