When Megan's getting dressed near the start of the film, she puts on her underwear and her top. By the time she has put on her top, she's also wearing jeans.
The window in Dr. Hill's door is broken when Dan and Meg go snooping around in his office, but it is whole later on when Dr. Hill returns.
Bone saws, like cast cutters, don't rotate, they vibrate.
In the very first sequence two signs can be seen next to the locked door. The first one reads 'Dr. Gruber Ghirurg'. The word 'Ghirurg' doesn't exist in German. The correct spelling is 'Chirurg' ('surgeon'). The sign below shows the three lines 'GEHIRN FORSCHUNGSL LABOR' ('brain research laboratory'). The first 'L' doesn't belong there. And as 'Gehirnforschungslabor' is in fact one word in German it should be hyphenated when wrapped.
A severed head wouldn't be able to speak, or breathe, without his lungs.
We see West shoving the drill through the cadaver's back and exiting his chest. The chest itself is a fake with the actor playing the cadaver holding on to it and shoving the drill through himself. We see this for ourselves when the two actors accidentally lift the fake chest high enough to reveal the real body underneath.
When West is using the bone saw on the reanimated zombie, the bottom of the artificial chest is visible on the bottom of the screen (full-screen VHS version only).
Just before Dr. Hill enters the padded room with the orderlies, he hits the window, making the whole wall shake. Again, revealing it as a fake wall.
When Dan is in the morgue at the beginning of the movie, you can clearly see the bag over the cadaver rising as the actor is breathing.
When West is stabbing Dr. Hill with the shovel, a hose is briefly seen shooting blood at his shirt sleeve.
No explanation is given as to how the (revived) severed head of Dr Hill can still make the rest of his separated body walk and open doors etc (despite a severed spinal cord etc) - both parts of him merely being revived wouldn't explain that.
When Herbert is experimenting on Dean Halsey, he says he's injecting 12cc of the reagent, but it's clearly a lot more.
When Herbert says he is upping the dosage on his first human test patient by 20cc, he is clearly injecting a lot more than that.