Visible crew/equipment: During the dream sequence, when Gilligan starts a showdown with Mr. Howell and the Professor, someone's face appears in the window in the background for a second, and it's none of the castaways - someone in the filming crew most likely.
Visible crew/equipment: As the Skipper makes an 'X' in the dirt, the shadow of the camera rig raising up is seen on the ground.
Visible crew/equipment: As Wrongway says "A couple more minutes, I could've been killed", the studio lighting is reflected on the left lens of his goggles.
Gilligan Meets Jungle Boy - S1-E20
Visible crew/equipment: When Gilligan is floating inside the balloon and then outside, the wires supporting him are visible. (00:19:00)
Little Island, Big Gun - S1-E18
Visible crew/equipment: At the beginning of the episode when Ginger comes out of the hut the shadow of the boom mic is visible on the hut.
Little Island, Big Gun - S1-E18
Visible crew/equipment: Gilligan shoots a bag of money in a tree with an arrow. As the money falls to the ground you can see a fan in the lower left corner of the screen aimed up towards the tree.
Visible crew/equipment: As the camera looks at Skipper and Gilligan, after the tossing of the sail, the edge of the studio tank is visible for a few seconds in the background.
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.