Factual error: The professional hitmen murder their victim in the hospital with a silenced .38 revolver. You'd think the mob would know better. Silencers have absolutely no effect on revolvers. They only work on closed-chambered, automatic weapons.
Factual error: An injured McGarrett is trapped in an overturned car, which emergency workers flip over before cutting him free. No rescue crew would ever do this: such violent movement could kill the patient. It's crucial that crash victims be extricated without causing further trauma: McGarrett would be freed leaving the wreck exactly where it was - upside down.