Revealing mistake: During Ash's fistfight with Henrietta near the end, the blade of the chainsaw can be seen flopping like rubber. (01:12:10)
Audio problem: It's quite obvious that all of the grunts and sounds of terror that Ash makes while his hand is attacking him in the kitchen are dubbed. (00:28:45)
Audio problem: As Ash backs up from the bridge, he squeals his tires on the dirt. (00:09:55)
Visible crew/equipment: When Ash and Annie hug, before the shelves crash down, you can see the head of a crewmember, crouched down behind the ledge, bobbing around. (01:11:05)
Visible crew/equipment: When Evil Ash picks up Jake, to throw him, from a ground-shot you can see a black cable attached to Ash's back, and to the right side of the screen a crewmembers hand comes into view. (00:56:15)
Visible crew/equipment: When Linda's headless body is chasing Ash through the tool shed, at some points you can see the stick that's holding her muppet-like body. Also, at one point you can see the hand holding it up. (It is very visible after she gets the chainsaw in her neck and Ash is trying to beat her away. Easiest seen in slow motion.) (00:21:00)
Continuity mistake: The hand gets it's thumb caught in a mousetrap, yet in the next shot it's index finger is the one caught in the trap. (00:31:40)
Continuity mistake: When Henrietta is attacking Ash, the clock in the background alternates from reading 1:15 or 1:30. (01:09:00)
Continuity mistake: When deadite Linda first pops up outside the window in front of Ash her hair is in a bun, but the view changes and her hair is down. (00:18:30)
Continuity mistake: During the movie, you can see that the metal handle of the chainsaw switches between being thin and thick. This is because they had replaced the handle half way through the shoot and had just painted the regular one silver and brown.
Other mistake: When Evil Ed picks up Jake and throws him headfirst into the lightbulb, watch the speed that he hits it and then falls away from it; he moves far to slowly.
Visible crew/equipment: When Henrietta is holding Annie up in the air you can see the shadow of the wire holding Henrietta on a ceiling rafter. (01:08:40)
Continuity mistake: Ash's shirt is untucked as he listens to the tape, but when he runs into the room after Linda screams, his shirt is suddenly tucked in. (00:03:15 - 00:05:20)
Revealing mistake: Evil Ash has no hand (just a stump) however when he grabs Annie and throws her into the wall, he has his right hand back. (01:01:10)
Factual error: In the "Groovy" shed scene, Ash holds a double-barreled shotgun in one hand and saws through the gun barrels with his newly-mounted chainsaw in about 2 seconds flat. He could no more cut through a hardened steel shotgun barrel with a chainsaw than he could cut through a marble counter-top with a butter knife.
Factual error: When Ash finds the bridge destroyed, in a side view we see the bridge missing, and both cliff edges either side of the screen. However the right cliff side is up MUCH higher than the cliff Ash is on. (00:10:45)
Continuity mistake: When Ash wakes up, after being knocked unconscious by the puddle, he has leaves stuck in his hair. In the next shot they've gone. (00:08:35)
Continuity mistake: Ash chops his hand off at the wrist, yet later it is seen to have a fair amount of forearm attached to it. (00:29:00)
Audio problem: After Linda's head is possessed and bites Ash's hand, as he's getting up and going to the work shed, his mouth doesn't move as he says the word "work-shed", and it is obvious that it is dubbed in. (00:28:55)
Audio problem: When Ash is struggling with Linda's head, he bangs it against several objects. At one point he slams the wall, which sounds like a large metal panel or piece of aluminum siding. The cabin is made of wood. (00:19:45)
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.