Revealing mistake: When the Professor comes over to the Robot to ask "How would you like to do us a favor?", look closely at the mesh like part of the Robot's "face", just left of the vertical black bar. You can see the Robot actor's eye blinking and looking around. When he nods, you can see the outline of the actor's head.
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.
Revealing mistake: In the closing credits of Seasons 2 and 3, after the camera pulls back to look at the castaways on the beach of the lagoon, in the upper left corner of the shot you can see the roofline and corner of a building.
Good Night Sweet Skipper - S1-E5
Revealing mistake: In the end scene, as Gilligan chases the bug, trying to quiet it, you can see the lines in the gourd where the hole will be after he hits Skipper with it.
Revealing mistake: As the gale force wind lifts Gilligan, you can see the outline of the harness under his clothes and can make out the wires holding him up.
Revealing mistake: When the shark takes a second bite of the raft, you can see the bite before it's taken.
Good Night Sweet Skipper - S1-E5
Revealing mistake: At the end, when Gilligan is holding to gourd up, chasing a fly, the lines of the cut out spot for the final gag are visible.
Revealing mistake: The beam the Skipper wants Gilligan to work on is six feet, but if you look carefully, the beam is already pre-cut.
Revealing mistake: In Gilligan's dream sequence, when the pirate goes to hit his fencing foil, the score mark where the break was supposed to happen is plainly visible, and he's holding it very still for the gag.
Revealing mistake: After the Skipper throws the attache case in the lagoon, it explodes. Problem is that the lagoon water and the water with the explosion are in two different areas, and where the explosion is, there was no case floating. Just markers floating where the explosive is placed underwater.
Pass the Vegetables, Please - S3-E3
Revealing mistake: As the camera pulls back from the hut, the bubbles can be seen coming from around the edges of the shot.
Pass the Vegetables, Please - S3-E3
Revealing mistake: As the Professor reads his meter, the bubbles are shooting from beside his nose, rather than his mouth.
Pass the Vegetables, Please - S3-E3
Revealing mistake: When Gilligan picks up a coconut, the first one he crushes easily is totally different than the ones in the pile it's on, showing it was a specially made prop.
Revealing mistake: After the Skipper tosses the sail into the water, look at the background. On the right side of the shot, the bottom of the backdrop is easily seen, showing this scene is filmed in a huge water tank set.
Revealing mistake: When the shark is attacking Gilligan's oar the second time, it stops completely for a couple of seconds.
Revealing mistake: As the shark goes by the floating bottle and towards the camera, the underwater rail the prop shark rides on is obvious.
Revealing mistake: While they are at the front or side of the damaged boat, you can see that the dark windows above them are obviously painted on, rather poorly in some spots, and not real windows.
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.
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.