After Dr. Graves cuts the bat open that he injects with the serum, the bat has a hole in its stomach. After it attacks him and Dr. Graves pins it down, there is no hole in the bat's stomach
At the end of Re-Animator, Dr Hill's head was crushed, the eyes were completely gouged and the head was thrown against a wall, however in this, the head is all but fully restored with no additional damage apart from still being separated from its body.
When Dr. Graves is battling the re-animated rat the words "Dr. Graves, Pathology" on his doctor's smock go from correct to a reversed image and then back to the correct, legible image.
When Dr. Graves is talking to Dr. Hill's head, there's a knock at the door. Dr. Graves hides the serum in a metal pan and covers it with a towel. When he goes back to talk to Hill, the serum is back next to Hill's head. It disappears again. You can also see the glow of the serum in the metal vice holding Hill's head, even though the serum is not in the shot.
Around 1:19, Francesca rushes to open the door and the curtain is closed. When she enters, the curtain is open.
Along with misspelling Mary Wollstonecraft Shelley's middle name as "Wollenscraft", her last name is also misspelled as "Shelly".
When Dan is trying to break down the door of the lab, the first time he slams into it, the two lights on either side of the door flicker, emphasizing the fake wall with the lights loosely attached.
The word "cemetery" is misspelled "cemetary" on the outside entrance wall.
In the end credits thank you, Mary Shelley's name is misspelled as Mary Wollenscraft Shelley, rather than Wollstonecraft.