Topsy-Turvy - S3-E10
Other mistake: At the end, after Mary Ann puts the pie down, and Gilligan drinks the juice, he hits the table, flipping the pie into the skipper's face. First, the Skipper starts to wince when Gilligan hits the table, preparing for the gag. Second, the pie tin defies physics by staying exactly in place on the board after the pie leaves it, and after the board bounces once and settles down. Third, Gilligan dips his finger directly into the cream on the Skipper's face, rather than missing a few times.
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.