Revealing mistake: After the Skipper tosses the sail into the water, look at the background. On the right side of the shot, the bottom of the backdrop is easily seen, showing this scene is filmed in a huge water tank set.
Revealing mistake: When the shark takes a second bite of the raft, you can see the bite before it's taken.
Revealing mistake: When the shark is attacking Gilligan's oar the second time, it stops completely for a couple of seconds.
Revealing mistake: As the shark goes by the floating bottle and towards the camera, the underwater rail the prop shark rides on is obvious.
Revealing mistake: The beam the Skipper wants Gilligan to work on is six feet, but if you look carefully, the beam is already pre-cut.
Voodoo Something to Me - S1-E4
Revealing mistake: While searching in the jungle, Gilligan slips and goes down a slope. As he gets toward the bottom, the plastic sheeting used to make the slide ripples.
Good Night Sweet Skipper - S1-E5
Revealing mistake: In the end scene, as Gilligan chases the bug, trying to quiet it, you can see the lines in the gourd where the hole will be after he hits Skipper with it.
Good Night Sweet Skipper - S1-E5
Revealing mistake: At the end, when Gilligan is holding to gourd up, chasing a fly, the lines of the cut out spot for the final gag are visible.
Revealing mistake: While they are at the front or side of the damaged boat, you can see that the dark windows above them are obviously painted on, rather poorly in some spots, and not real windows.
Revealing mistake: As the gale force wind lifts Gilligan, you can see the outline of the harness under his clothes and can make out the wires holding him up.
Revealing mistake: When Mr. Howell and the Professor are thrown by Gilligan and the Skipper, during the struggle you can see Gilligan flying in from the right side. But it can easily see that it is a dummy in Gilligan's clothes.
Answer: Bob Denver answered this when he was on a talk show hosted by Pat Sajak. He said they were going to write into an episode where the castaways found all these clothes, but it got too complicated, so he said, they decided, "who cares, it's only a TV show," and forgot about it. So the writers and actors even asked this, they just never bothered to address it.