Deliberate mistake: When the hillbilly is dragged into the cellar, the resulting explosion of blood has a much greater volume than the actual amount of blood in a human body. This is done to support the campy gore motif of the film. (01:03:20)
Revealing mistake: After Ash is punched unconcious by Jake, as Annie shakes him, screaming at him, he grabs her arm, although he's out cold. (00:36:25)
Continuity mistake: After Ash opens the Book of the Dead, he doesn't swaps pages, yet in the second shot the pages the book is open on are different. (00:02:50)
Continuity mistake: After Ash wakes up by the puddle of muddy water, the amount of mud on his face and neck differs. (00:06:40)
Audio problem: When Ash is standing by the self-playing piano, it is clear that the keys being pushed are way out of sync with the music. (00:14:25)
Revealing mistake: When Ash is fighting the headless corpse in the shed, in one shot the chainsaw bends clearly. (00:20:10)
Continuity mistake: After Ask looks in the mirror, as his hand starts to turn evil, green slime has appeared on Ash's shirt. (00:22:40)
Continuity mistake: When Ash lays on the floor, after his evil hand has attacked him, the wrappers, boxes, smashed china etc. around him moves around between shots. (00:26:35)
Continuity mistake: When Ash hacks Evil Ed, the clock reads 11:10. Later when the clock is shown stopping, it is 10:40. (00:43:40 - 00:45:00)
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.
Continuity mistake: The mousetrap is visible outside of the hole in the wall, before the hand gets it's thumb stuck in it and leaves the trap outside the hole. (00:31:25)
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).
Other mistake: When Ash decapitates Henrietta with the chainsaw at the end, watch very closely: her head comes off about five centimetres BELOW where Ash hits her.
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)
Continuity mistake: When Linda's head bites Ash's hand, he starts trying to hit it off. He runs to the mantle and hits the head, knocking a green candle off the mantle. In later shots it's back on. (00:18:45 - 00:31:10)
Continuity mistake: When Ash enters the shed to get Linda's head off of his hand, the position of the vice's lever changes between shots. (00:19:35)
Continuity mistake: After the chainsaw was embedded in Linda's neck, we see black blood spray everywhere. After the headless body dies, Ash takes the chainsaw, which is completely clean. (00:20:05)
Revealing mistake: When Ash stomps the trapdoor onto Henrietta's head and her eye flies out, the closeup reveals that the "eye" is something like a styrofoam ball with the veins painted on.
Revealing mistake: As Ash is duct-taping his sleeve shut after he cuts off his hand, you can see the impression of his largest knuckles and first two fingers under the cloth. (00:31:45)
Answer: Sam Rami could not get the footage of Evil Dead to use in the sequel, so he remade the movie in a shortened form for the beginning. Since Linda was the only original character other than Ash to come back he included her in the recap but deleted the others as not to waste time. An in universe answer could be that trauma of losing his friends made Ash block them out and rewrite history in his head, except for Linda who comes back to haunt him in the movie so he is forced to deal with it.