Other mistake: All of their sheets were the same type. The sheet that the "Ghost" supposedly took from them was more silken, not the regular knit.
Revealing mistake: After Gilligan lies down on his hammock, there is an "X" in the sand where actor Alan Hale, Jr. stands. Obviously it is the mark where Hale is supposed to stand, and he does stand there as he speaks to Bob Denver.
Visible crew/equipment: Outside the Howell's hut as the castaways ready to leave the island, the shadow of a boom mike enters the upper left side of the scene twice.
Continuity mistake: When Skipper tries on the sheet to teach Gilligan not to be scared, we see him putting it on and it is solid white. However, when Gilligan says he isn't scared he says it is because a "ghost doesn't wear a sheet that says S.S. Minnow on it." Now the sheet has wording on it. (00:08:45)
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.