Revealing mistake: Any time you see the vines used for different purposes, it's painfully obvious that they're very poorly disguised rope, such as the ones the Skipper and Professor use to rock the boat mock-up in the episode "Quick, Before It Sinks."
Revealing mistake: When the Professor comes over to the Robot to ask "How would you like to do us a favor?", look closely at the mesh like part of the Robot's "face", just left of the vertical black bar. You can see the Robot actor's eye blinking and looking around. When he nods, you can see the outline of the actor's head.
Revealing mistake: In the closing credits of Seasons 2 and 3, after the camera pulls back to look at the castaways on the beach of the lagoon, in the upper left corner of the shot you can see the roofline and corner of a building.
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.
Revealing mistake: As the Professor and Skipper get the test vehicle rocking faster, there are a few times that the metal poles built into the table legs to slide it is visible. Then as Gilligan slides on the table, the channels the poles slid along are visible, and it's obvious that the figure flying out into the men's arms is a dummy.
Gilligan's Living Doll - S2-E21
Revealing mistake: Watch the window on the barrel used as a washer. It looks to be somebody inside wiping the glass to simulate clothes washing.
Operation: Steam Heat - S2-E25
Revealing mistake: After Gilligan says his line about the earth swallowing them all up, an explosion rocks the area. Between the end of the line, and the rocking, there's a split second of blackness indicating a cut between the line and the action.
Operation: Steam Heat - S2-E25
Revealing mistake: When Gilligan and Ginger are in the cave at the volcano, you can see that the steam in the two passageway openings behind them is being shot in from jet nozzles.
Will the Real Mr. Howell Please Stand Up? - S2-E26
Revealing mistake: When Gilligan demonstrates his man-made wings, when the camera goes close up to him, you can see the wires.
Revealing mistake: After Gilligan lies down on his hammock, there is an "X" in the sand where actor Alan Hale, Jr. stands. Obviously it is the mark where Hale is supposed to stand, and he does stand there as he speaks to Bob Denver.
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.