When the professor is using the guillotine, you can see the (Styrofoam) coconuts are precut down the middle.
At the end when the spear gun fires during the reenactment, the spear somehow pierces Gilligan's lab coat above him, leaving him dangling from the pole.
The round Life Preserver on the wall inside the hut had SS Minnow II written on it, when the castaway were re-enacting the night where they each had dealings with the murder victim. The boat was always known as the SS Minnow or SS Minnow I.
It is inconceivable that all 7 of the castaways could have known Randolph Blake before the shipwreck, being that they were all strangers to one another, with the exception of the skipper and Gilligan, and Mr. and Mrs. Howell, let alone fish a crate out of their lagoon that contains newspapers telling of Blake's murder.