Other mistake: Twice in the episode it is explicitly said that Magnum's phone has been disconnected, and that Higgins checked with the phone company. Yet at the end, it turns out 'the lads' chewed on the cable. Surely the phone company wouldn't tell the estate manager that the line has been disconnected for billing problems (so, a well determined administrative issue with proper paperwork done) when it's a wiring problem.
Other mistake: Magnum runs away to chase the sniper. A few extras promptly start yelling and screaming despite the fact that from their position they can't have properly seen the victim of the sniper yet (and there's no blood). (00:14:50)
