Stupidity: Given Laurie Strode survived an attack by a still on the loose maniac she's not guarded at the hospital?
Stupidity: When Dr Loomis and Sheriff Bracket are chasing who they think to be Michael Myers, you can tell that it isn't him. Michael Myers is supposed to be 6'2 - Ben Tramer is maybe 5'4. If Michael was shot 6 times there would be blood on his overalls. And the mask that the kid is wearing has blonde hair, whereas the mask Michael wears is brown.
Suggested correction: Remember that Loomis has only seen Michael Myers in his mask once, for a few seconds, in darkness. Loomis' reaction is still pretty hysterical and arguably stupid, but not necessarily for that reason.
Suggested correction: Michael in the original was 5'10", and in Halloween II the actor playing him was 5'9." Ben Tramer was a high schooler and could have easily been 5'10." Loomis was also extremely paranoid and almost hysterical at finding Michael, and could have not noticed the hair or non-bloody coveralls in his frantic search for Michael.
Stupidity: The sheriff kneels down to examine Michael's 'body' as the doctor yells at him to get away, and the sheriff tells him Michael stopped breathing. Michael suddenly grabs the sheriff and throws him onto the ground and raises the scalpel to kill him. The doctor aims his gun at Michael to shoot him but doesn't, when he had the opportunity before the sheriff is killed.