Continuity mistake: When Freddy rendezvous with the gang, Jen's hair when she is applauding at the back is tied in two scruffy bunches sticking out. A couple of seconds later, she has only one bunch tied. From then on, it is tied in two bunches. (00:21:30)
Continuity mistake: When Michael Myers is holding the tripod to impale the crewman, the length of the sharpened end changes in length and shape numerous times. First, it's wider and smaller. Then it's slightly longer and narrower. And when it's finally through the victim, it's long enough to go through the neck and into the adjacent wall.
Continuity mistake: When you see Deckard is in his room with his friend the second time Deckard is relaxed,leant back on his chair with his cheek resting on his fist but in the very next shot he is typing on his computer. (00:25:25)
Continuity mistake: Within a few minutes after entering Michael Myers' house Sara and Rudy go into the kitchen. Sara opens the door and a baby-chair fold out quickly, afterwards when they all take a look at it, there are no hinges or screws, etc. to make it flip open. (00:34:05)
Continuity mistake: When Michael is walking towards Laurie's room in the sanitarium, the window on the door is yellow as if a light is on in the room. Then in the next shot it is blue as if a light was not on.
Revealing mistake: At the end of the film the garage is on fire and just before Freddy bursts through the door, there is a really quick close up shot of Deckard kicking down the door. (01:16:30)
Continuity mistake: In one of the last scenes where Sara Moyer has a blanket around her shoulders. She has her palm pilot in her hand and she has just received a message from Berman saying that he can see her on the news. The very next second after reading that entry you can here the palm pilot shut so you know she a has closed it. Then the very next shot you can actually see her shut the palm pilot.
Answer: At first impression, Laurie was catatonic not only from the shock of killing an innocent man, but also from a lifetime of being chased down by her murderous brother. She obviously knows from past experiences that nothing she does (including faking her own death) will stop Michael from finding her. It is later revealed she has been faking her mental issues (think when she has been hiding her regular meds in the raggedy Ann doll) and is committed only to stopping Michael, no matter what the cost. She may have also been trying to protect her son and the only way to do that is to allow herself to be institutionalized in order to lure Michael there.