Gilligan's hand with the rock swings back and knocks the door with the life saver hanging on it off the hinges and flying outside. Then Skipper walks up wearing the door around his neck where it has landed with the life saver on the outside but in the next shot when the Skipper removes the door the life saver has changed positions and is now under the door.
It is unclear why the castaways are using rocks as bowling balls rather then the cannon balls they found in "Plant You Now, Dig You Later". Maybe it's because they were able to make a more regulation sized bowling set sometime in between.
When Gilligan, now invisible, supposedly drinks the contents of a glass in MaryAnn's hand through a straw, it empties out, but a small hose attached to it coming out of her sleeve is clearly visible.
When the Skipper hands an invisible Gilligan a handheld mirror, it's clearly hanging by invisible string based on how it acts when the Skipper lets go of it.
After the bowling ball has been blown off Gilligan's hand and the professor is picking through a pile of branches to find him, the bowling ball scoots several feet across the ground just from being touched by a branch - making it obvious that it wasn't actually made of stone but was just a lightweight prop.
It's clear that Gilligan is pulled away by strings when he is hit by lightning the first time, and that the lightning is a graphic.
After removing the palm fronds and discovering Gilligan is invisible, the Skipper's fly is open.
Gilligan's invisibility would apply only to his physical body, not his clothes.
The Howells are shown wearing yellow rubber gloves and, Mrs Howell, a white lab coat. Ginger is shown wearing a white nurse's uniform and hat; nurses' hats were unique to the school they came from. It's unclear how they happened to have these items on hand.