Our Vines Have Tender Apes - S3-E20
Audio problem: Towards the end of the episode, the castaways read Tongo's note that he can't take them back with him due to how he behaved with the gorilla. Mr. Howell yells "we won't tell anyone!" followed by other random comments from the others for Tongo to come back. Tongo leaves and as the camera pulls away from the castaways, the same audio repeats itself even though this time doesn't match what anyone is saying.
Gilligan's Personal Magnetism - S3-E21
Continuity mistake: When Gilligan knocks the door off the hut it lands on the Skipper's neck with the life preserver on the top. In the next shot the preserver is on the bottom.
Gilligan's Personal Magnetism - S3-E21
Visible crew/equipment: Toward the end, the group is around the table, talking about Gilligan. Then they see a pear and a bunch of grapes float over to a plate at the head of the table. Skipper goes over and addresses the invisible Gilligan. If you look at the top of the hut, you can see the wires used to make the fruit float. Also, earlier in the hut with Ginger, when Gilligan bit the pear, you can see the wire holding the pear as it shakes after the bite.
Splashdown - S3-E22
Plot hole: Most of the episode focuses on just Gilligan and the Skipper riding the capsule out to sea (although the Howells and the girls attempt to stow away with them at first). However the whole plot changes at the end... suddenly with nobody aboard the capsule, Gilligan tells the Skipper it floated off and Skipper yells at him saying "We could have ALL been on that capsule!" when it suddenly explodes thus saving everybody as if the plot now called for everyone to have been on board. It was still supposed to be Gilligan and Skipper only.
The Second Ginger Grant - S3-E24
Revealing mistake: When "Ginger" (Mary Ann) forgets the words to the song, she asks the maestro, Mr. Howell, to restart the record. Before and after he puts the needle on the beginning, you hear the music, but the turntable isn't moving.
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.