Revealing mistake: As the Professor and Skipper get the test vehicle rocking faster, there are a few times that the metal poles built into the table legs to slide it is visible. Then as Gilligan slides on the table, the channels the poles slid along are visible, and it's obvious that the figure flying out into the men's arms is a dummy.
Gilligan's Living Doll - S2-E21
Revealing mistake: Watch the window on the barrel used as a washer. It looks to be somebody inside wiping the glass to simulate clothes washing.
Operation: Steam Heat - S2-E25
Revealing mistake: After Gilligan says his line about the earth swallowing them all up, an explosion rocks the area. Between the end of the line, and the rocking, there's a split second of blackness indicating a cut between the line and the action.
Operation: Steam Heat - S2-E25
Revealing mistake: When Gilligan and Ginger are in the cave at the volcano, you can see that the steam in the two passageway openings behind them is being shot in from jet nozzles.
Will the Real Mr. Howell Please Stand Up? - S2-E26
Revealing mistake: When Gilligan demonstrates his man-made wings, when the camera goes close up to him, you can see the wires.
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.
Revealing mistake: When Skipper is blindfolded, as he rears back getting ready to sneeze, watch his right shoulder. You can see the line that pulls the blind off when he sneezes.
Revealing mistake: When Skipper is sneezing after Gilligan showered, he turns towards the wardrobe. As he sneezes, in a "blink and you miss it" moment, the line pulling up the bottom of the green cloth is momentarily visible, pulling from the upper right corner of the screen.
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.