Just prior to meeting the Black Orchid for the first time, Magnum emerges from the sea wearing only swimming shorts. In the middle of the conversation with the Orchid and Higgins, he is suddenly completely dry and wearing a blue vest (t-shirt) and red baseball cap.
At the picnic when the two masked men attack, one of them fires a shot at the Ferrari and puts a hole in the windshield. But when Louise starts to drive there no longer is a damaged windshield. Another tiny error is that the bullet also should have struck the rear window or pillar, but it apparently disappeared into thin air.
When Magnum drives off after talking to Christie at the Jackson's home, he accelerates down the driveway as Mr. Jackson's limousine approaches and is forced over to.the left side. This would mean Selleck was driving on the wrong side of the driveway.