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).
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.
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.
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.