Gilligan Meets Jungle Boy - S1-E20
Continuity mistake: When the Skipper squeezes Gilligan to get rid of the helium, in one shot the Skipper's hat is on, and in the other it is not.
The Return of Wrongway Feldman - S1-E25
Other mistake: As the plane taxis in, there are tracks from the wheels in the sand prior to the plane's entry into the scene, showing a previous take.
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 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: 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.
Other mistake: As the Skipper is on the ground under the tree waiting for Ginger to get Gilligan, the headhunter is crawling behind him. You can see some smoke in the lower left corner, likely cigarette smoke from a crew member.
Other mistake: As Mary Ann pedals to power the sewing machine, the bobbin only clicks back and forth, giving the illusion of real movement.
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.