Revealing mistake: When John Cusack is driving the actress, there is a shot looking over his right shoulder at the dashboard. The speedometer reads 0 mph. (00:08:15)
Revealing mistake: When Ed is driving the limo at the beginning of the movie, the gearshift is in the park position.
Revealing mistake: When the guests open up the freezer containing the dead hotel clerk, it falls towards the camera, i.e. the viewer. Right before it changes to the reaction shot of all the guests, the corpse blinks. (00:52:50)
Revealing mistake: Near the end of the film when we see Rhodes shoot Larry in the chest, you can see an outline of something underneath his vest, very obviously a device/squib simulating being shot in the chest. (01:15:15)
Revealing mistake: When John Cusack notices Timmy's mom is dead, you can see her still breathing. (01:01:35)
Revealing mistake: When John Cusack is coming out of the motel grounds looking for Jake Busey, the gate is banging away furiously from the wind but the rain is coming straight down. The rain should be sideways if the gate is banging so much from the wind.
Revealing mistake: When the woman Ed hit dies, her eyes move. (01:00:30)
Answer: The actual, "real-life" killings happened before the events shown in the movie and are shown briefly in a flashback. The authorities knew that Malcolm had done it while believing to be one of his alternate personalities, but did not know which one, and had to be convinced that the killer personality was dead before changing his sentence to life imprisonment. Meanwhile, the killings we see in the motel during the movie, are Malcolm killing off his other personas, leaving in fact only - the killer.
Twotall