Deliberate mistake: During the final battle, when Ash is driving around in his car with the giant fan mounted to the front mowing down enemies, some of the evil skeletons he runs into aren't moving at all for several seconds before being hit. This reveals that the were just props designed and set-up to be run over for the effect. Additionally, the first two sets of skeletons he chops up in the fan are the exact same skeletons shot from different angles. (You can tell from the distinct way the one skeleton's arm comes off and ends up on the car's front).
Other mistake: You can see that Ash is without his shotgun after being captured by the knights, even when he's thrown into the pit. However, it appears out of nowhere when he is prompted to shoot Arthur's sword. The only weapon that we saw returned to him was the chainsaw when the wise man tossed it down to him.
Factual error: When all is back to normal and Ash is back at his job at S-Mart, a witch shows up and starts trashing the place. Ash is knocked behind the sporting goods counter, where there is (of course) a cabinet containing guns. Ash then jumps up and blows the witch away, firing 37 times. He's using a .38 caliber Winchester Model 1892 Saddle Ring Carbine with a "Trapper" loop lever. It only holds 9 rounds.