Continuity mistake: While the castaways are standing around while their clothes dry, Mary Ann is not wearing any shoes. As the scene continues she is shown standing wearing black shoes. (00:05:00 - 00:06:00)
Other mistake: The Communal Hut is built in the clearing, and has a dirt floor. When Skipper and Gilligan check outside, it's floating in the lagoon. But that suggests it had a wooden floor.
Continuity mistake: As Mr. Howell tells his story, the table Ginger is on rocks to the point the canteen beside her falls off. A second later, she's drinking out of it.
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.
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.