Deliberate mistake: During the sting attempt towards the beginning, Douglas confronts Snyder on the roof of the building. A helicopter rises up from nowhere without making any sound prior to that.
Deliberate mistake: Ash's car breaks out into the courtyard and the whirling, windmill thing batters the Deadites aside. We see the carnage a couple of times filmed from different angles, one Deadite, planted, legs apart, gets battered twice and the guys who come in from the left and are quickly jerked off screen are shown again seconds later.
Deliberate mistake: During the Hall of Fame sequence at the end, Dottie sees the display about Jimmie hitting 58 home runs and there is a long wide shot of it as she stands next to it. Then there is a close-up of the bottom of the text which says born 1906 - died 1987, which wasn't present during the wide shot a moment earlier. The filmmakers likely saved this for the close-up for dramatic effect, but by blocking the initial shot without that part obscured it makes the text magically appear out of nowhere.
Deliberate mistake: When Wayne begins playing the "Excalibur" guitar, the clerk stops him and directs him to a "No Stairway to Heaven" sign. The problem is that the guitar riff Wayne started playing sounded nothing like "Stairway to Heaven" so the clerk had no reason to interrupt him. Evidently the intro to "Stairway" was actually played in the theatrical version of the film, but its exclusion in the home video release makes the scene rather dubious.
Deliberate mistake: When the nuns sing for the first time in the church in perfect tone then break into the dance version of the song, the reverend mother acts like it's the first time she has heard it. In reality, she would have heard the rehearsals as the convent is very quiet and you can hear the choir rehearsing when she tells Deloris she will join the choir.
Suggested correction: She may have heard it, but never seen it. What startled her were the moves.
Deliberate mistake: Gonzo and Rizzo are selling apples in the beginning, including the Red Delicious cultivar. That wasn't invented until the 1880s and the story is set sometime in the first half of the 1800s. Most likely intentional as they are the narrators.
Deliberate mistake: When Marge and Dick are being beamed in outer space, there's no way they could be breathing because there's no breathable air in space.