Revealing mistake: In the first shot with Magnum on his surf ski, Tom Selleck is replaced by a body double whose face can be seen distinctly behind the large wave. (00:02:40)
Continuity mistake: TC is driving his team of youngsters for the 4th of July game; in the aerial shot of the van, it is driving a different road than the one shown in the interior view, and it's driving on the leftmost lane while it had cars on both sides of it and the right railing has been closer. (00:08:00)
Factual error: In the flashback of 6 years-old Magnum paddling with his dad before he leaves for the war in Korea, Mr. Magnum is wearing a Rolex GMT Master with the Pepsi bezel, a model designed in 1955, well after the war. (00:12:05)
Revealing mistake: When TC and the kids are watching the game live at the stadium, the film is much grainier making excessively obvious the usage of stock footage. (00:17:00)
Visible crew/equipment: During the flashback, while Magnum is on the longboard paddling five miles to the island, the chopper is flying over the open ocean when TC starts talking to himself about Magnum bucking for a section-8, and as the chopper's nose moves upward the reflection of set lighting, etc, can be seen on the windshield. (00:17:05)
Continuity mistake: Agatha is consoling Higgins for the loss at polo. She tells him that it was good strategy as she puts the pot down and puts her hand on the teacup. Next shot and as she elaborates she is holding nothing and putting her hand down to get the cup just then. (00:19:35)
Continuity mistake: During the chess game, Higgins does a Magnum on Agatha with his "I know what you're thinking...", causing her to raise her hands already at the very end of that specific shot. She will start the action all over again only later in a subsequent shot. (00:27:35)
Continuity mistake: When TC looks at a passing Ferrari and thinks about Magnum, the woman in close-up has her hair heavily windblown, while it's mostly still in the wider angle. (00:29:00)
Continuity mistake: TC gets anxious after passing by "She-Magnum" and honks at the cars in front of him at the traffic lights. It's obvious from the light and the buildings that the traffic lights shot is not matching the other views. (00:29:10)
Other mistake: Rick mentions "It'll take us half the night to limp home and I don't want to go through the reef in the dark with one engine." He's lying because he wants to spend the night with her, but even considering that, the fact that there are islands behind him as he speaks, but through all the rest of the scene both act as if they were in the middle of the ocean (she does not object, he looks out to sea with his binoculars), is a spoiler. (00:30:30)
Continuity mistake: Magnum's dad says on tape "I'll be home by the 4th." Both Magnum and his mom cheer. In close-up on Mrs. Magnum as the tape says "I hope so", Mini-Magnum is casting a shadow on her, but he was at greater distance in the previous shot, no shadow. (00:36:50)
Continuity mistake: TC and Higgins look into the guest house. From the top of the stairs looking down, there's a newspaper that is perfectly perched on top of the lamp shade. When they get downstairs and Higgins muses about abhorrent acts of perversion involving lizards, the sheet got limp. (00:37:45)
Revealing mistake: When Magnum screams to attract Rick's attention and you can see the whole yacht, you can spot the real driver of the boat on the flybridge, not lit. (00:41:00)
Other mistake: Magnum is 5 years old when his dad leaves for Korea, and is 6 by the time he's dead, coming back in a coffin on July 4th 1951. That does not work with the birthdate that appeared on the driver license in Memories Are Forever, episode 2-5, which is even abundantly referenced here with footage being re-used (Magnum's wedding party with bombs, and making love to his wife years later in the guest house). He was born January 5th 1946 in the document shown in close-up there.
Other mistake: Rick breaks off his hot date and goes searching for Magnum driving his boat for a long while -before- contacting Higgins to see if Magnum is missing at all. It also is the middle of the night and close to dawn (Magnum has to wait just one hour before the sun comes up) but Rick is still drinking wine (but not drunk) and hasn't even touched second base with the girl despite the fact that they were mixing wine and going below when the sun was still up.
Other mistake: Higgins says he hasn't seen Magnum (or his surf ski) "for two days", but Magnum left on the morning of the 4th, and has been in the ocean for 20 hours total. He spends the 4th on his own, but on the 3rd his friends and the ever-seeing majordomo of the estate should have gotten any sign of his presence.
