Continuity mistake: When the load of lumber is dropped on Captain Greer's 1968 Ford Galaxy 500, the car turns into a beat-up 1960 Oldsmobile just before it is squashed.
Continuity mistake: 9-4 "Man on Fire": McGarrett's blackboard already lists plutonium as the poison source before Dr. Ormsbee enters to inform him that plutonium was the poison source.
A Sour Note - S2-E2
Factual error: Gino commits a murder using a six-shooter with a silencer attached, and his shot doesn't make any noise. But it should have. Though it's a long-standing Hollywood cliché, a revolver with a silencer wouldn't be silent at all. Unlike automatic weapons, revolvers are open-chambered, which allows sound to escape. They can't be muffled with a silencer. (00:34:00)
Visible crew/equipment: When Reed asks Walters how many kids he has, the moving boom mic casts a shadow on the lockers at the top, left side of the screen. It appears again a few shots later. (00:00:50)