Factual error: As Ash is hunting his rebellious hand, he blows a hole about the size of a fist in the wall, then removes two spent shells from the shotgun. At that range, a double-barreled shotgun blast would have dealt much more damage. At several later points, he deals the exact same damage to a wall with only one barrel. (00:32:45)
Revealing mistake: Ash faces the oval, wall-mounted mirror and tries to reassure himself that he's fine, until his mirror-image lunges out and tries to strangle him. Throughout this scene, the background set and lighting "reflected" in the mirror changes very noticeably between the real mirror and the hole-in-the-wall practical effect. Additionally, the Ash that lunges through the mirror is NOT a "mirror-image" of the other Ash, as the blood patterns on their faces are distinctly different, which is obvious in the face-to-face profile shot. For that matter, the blood patterns on Ash's face are inconsistent all throughout the film, frequently from one shot to the next.
Revealing mistake: When Evil Ed tears off some of Bobby Jo's hair and eats it, the head is made out of latex (Watch for how easily everything bends on his face, even the jawbone bends at one point).
Factual error: A line of blood flies onto the door of the work shed while Ash is using the chainsaw on Linda's head. However, the shadows of Ash and the head show that they are perpendicular to the door. All of the blood would be moved by the chainsaw in the direction of the cut, either toward Ash or the wall behind the head, so there is no way that much blood could land on the door. (00:22:00)