The Secret of Gilligan's Island - S3-E25
Other mistake: At the end, the Professor and Skipper walk up on Gilligan, who's busy chiseling a message on a stone. Look carefully, because he's hitting the chisel, but the tip never touches the stone, and the 'E' in 'HOME SWEET HOME' (the message Gilligan is working on) is plainly visible.
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.