Continuity mistake: There are several shots of the island used in the series. In the opening, it appears small. In some inland shots it appears immense and mountainous, despite being described numerous times as a "tiny island somewhere in the Pacific."
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."
Plot hole: Many episodes show either visitors on the island swimming away or objects floating away on rafts (a lion, a silent film, etc) that at the end of the episode always make it safely to civilization (as heard on the radio)... yet, none of the castaways ever could swim or leave on a raft to attempt to get off the island and get rescued in the same way.
Plot hole: On many different occasions, the castaways meet a wide variety of people who say that they have been on the island for either ten or more years thinking they were the only ones on the island before the SS Minnow even arrived. With so many people who had been on the island for such an extended time, it seems kind of strange that none of them had ever met one another.
Visible crew/equipment: When Gilligan puts the Skipper down, you can see the wires attached to the corners of the lounge seat.
Continuity mistake: In the opening credits, the boat used as the Minnow changes. It goes from having an upper steering deck, to flat topped and different colored, and back.
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.
Factual error: The portable radio is an AM band only, therefore there is no need for the external telescoping FM antenna.
Plot hole: How could it be that every time someone or something was visiting the island during many episodes, they always made it back to civilization by the ending scene (as heard on the radio)? I can understand an actual person, but once anything else (such as a lion, a movie reel on a raft, etc.) leaves the island, it arrives perfectly by just simply slowly floating on the water with no human to guide them. Maybe the one exception could be the robot (programmed by the Professor).
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.