Other mistake: When Ash is splitting into his two halves, he backs into a huge tree and it moves. The tree moves again a few minutes later when bad Ash is shot and flies into it. (00:37:45 - 00:39:00)
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.
Other mistake: When Ash is separating into two (good and evil) Evil Ash is growing on his right side which is the same side as his "gauntlet" hand so the Evil version should have it, not Good Ash.
Other mistake: Just after his 'If any of you primates even touches me' speech, Ash yells and then turns to shoot the pit monster. Only problem is, while he's yelling, Ash is turning to his right. But in the next shot, he's turned to his left. (00:18:45)
Answer: Towards the end of the film, when Ash mounts his horse and rides off into the sunset (before it then cuts to present day and he's back at the S-Mart), you can see his chainsaw is strapped to the horse's saddle. It seems that he either recovered it after the battle or perhaps he sent someone to the windmill to retrieve it.