Revealing mistake: Near the end of the episode, all the castaways are passed out cold when Lord Beasley finally catches his butterfly. Right after saying the line "That's the trouble with you Americans - you simply don't know how to hold your tea" as he's exiting off camera, Mrs. Howell starts to open her eyes slightly, but then quickly shuts them again. Perhaps she thought the cameras stopped rolling.
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.
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.
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.
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.
Pass the Vegetables, Please - S3-E3
Revealing mistake: When Gilligan tosses the coconut, as it flies past the Skipper, the line it slid on catches the light as it shakes.
Pass the Vegetables, Please - S3-E3
Revealing mistake: After the Professor puts everyone to sleep reading the book, the camera backs up, showing the straight seams of the soundstage floor where the set ends.
Pass the Vegetables, Please - S3-E3
Revealing mistake: The guide wire used to "throw" the coconut through the hut by Gilligan is visible.
Revealing mistake: In the dream sequence, the boss, E.V.I.L., is bald. You can see the line of the skin wig he has on.
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.
The Hunter - S3-E18
Revealing mistake: As Gilligan sits in front of a large boulder, the disguised charge that simulates a bullet strike is easily seen.
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.
Hair Today, Gone Tomorrow - S3-E8
Revealing mistake: After Skipper goes bald, you can see the line of the skin wig on him.
Hair Today, Gone Tomorrow - S3-E8
Revealing mistake: When Gilligan is bald, you can plainly see the line of the skin wig, as well as able to see his hair under it.
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."
Pass the Vegetables, Please - S3-E3
Revealing mistake: When Gilligan is lifting the Skipper in the lounge chair, you can see the wires used to raise the chair up when the camera is close up on the Skipper.
Revealing mistake: The marks where Jim Backus and Natalie Schaffer are supposed to stand are some unnaturally positioned twigs in the sand during the scene of Mr. Howell's practice polo pony stuck in the tree.
Revealing mistake: Whenever the castaways are at the lagoon, there is sun shining with actual shadows from the sun - yet whenever by the huts or anywhere else other than the lagoon, there is never any sun or shadows from a sun visible. It's clear the lagoon area is filmed outdoors and indoors using studio lighting for everywhere else.
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.