Other mistake: After Gilligan gets up to take the sand to Mary Ann, the camera closes in on the box lid. After it does, the picture goes into a freeze frame because the ripples in the water of the lagoon stop dead still for the few seconds until the dissolve into the next scene.
Other mistake: At the end, after Mary Ann puts the pie down, and Gilligan drinks the juice, he hits the table, flipping the pie into the skipper's face. First, the Skipper starts to wince when Gilligan hits the table, preparing for the gag. Second, the pie tin defies physics by staying exactly in place on the board after the pie leaves it, and after the board bounces once and settles down. Third, Gilligan dips his finger directly into the cream on the Skipper's face, rather than missing a few times.
Other mistake: Mary Ann comes up to the Skipper while he's learning to juggle. If you look closely, the coconut bounces on the ground a couple of inches from his left foot. When Gilligan approaches, the coconut bounces off the bottom shelf. And in both times, the Skipper reacted in pain.
Other mistake: At the beginning of the episode when the Skipper wakes up on the boat this scene is actually from the original pilot, as you can see the 3 different actors that played the part of the professor, Ginger, and Mary Ann. (00:01:20)
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.