Plot hole: We see Michael wearing the mask throughout the day, from morning at the Myers House, to mid-day outside Laurie's school, to afternoon when Tommy is leaving school, yet when Laurie and Annie see the sheriff in front of the hardware store on the way to their babysitting jobs (presumably in late afternoon or early evening), the store's alarm is still ringing and the sheriff mentions that one of the items stolen was a Halloween mask.
Plot hole: Immediately after the scene where Laurie is talking to Linda while she is being strangled by Michael, Laurie looks out the window and sees the LOWER level of the other house lights turned on. Michael was on the upper level and could not have gotten down the stairs to the first level in time (1 or 2 seconds) to have turn on the lights.
Plot hole: When Laurie and her friend see the Sheriff in front of the hardware store, he tells them someone broke in and stole "A Halloween mask, some rope, and a couple of knives." Without someone doing a complete inventory of the store, there would be no way to know the specific items that had been stolen, especially when they are such commonplace things as knives and rope in a hardware store.
Plot hole: Since the station wagon from the asylum was stolen, it should have been reported stolen along with the escape of Mike Myers. So why was the sheriff department not looking out for the stolen station wagon? We see Myers driving that station wagon freely without being stopped, while wearing a mask in broad daylight.