Lowenstein states that oral sex is not illegal in Florida. Florida statute 800.02 made "unnatural and lascivious acts", which included oral-genital contact, a misdemeanor until all sodomy laws were struck down by the US Supreme Court in 2003.
After he and Matty have sex for the last time, Edmund rolls over to his side of the bed covered in sweat and hair matted to his forehead. Seconds later when he hears noises downstairs, he springs from the bed completely dry with no indications of his prior activity.
Just before seeing the clown driving a car, Ned Racine walks into reflected sunlight. This light causes the camera and crew to cast a large and distinct moving shadow on the road next to his convertible.
At the end of the movie, as Ned thumbs through the yearbook, the way he flips through the pages is wrong. He looks at "Tyler" first, then flips the page forward for "Simpson". Since "S" comes before "T", he should have flipped the pages backwards to see the "Simpson" photo.
When Heather discovers Matty performing oral sex on Ned in the garden, instead of seeing her head where it should be, she is clearly seen crawling away from him, well before Heather exclaims, "Aunt Matty."
The goof items below may give away important plot points.
When the new will is invalidated, Edmund's death is determined to be intestate. Under the doctrine of dependent relative revocation, the first will written by Edmund would be valid. As a result, Heather would have received 50%, as stated in the first will.
Under Florida law at that time, the trust would not yet have been void as stated in the movie. Florida had a "wait and see" provision from 1979 to 1988 which made the gift valid unless it failed to vest within lifetime of someone plus 21 years.
When Ned looks through the boathouse window after hesitating to go in to get the glasses, he sees twine supposedly running from the door-knob into the darkness of the room. The angle of the rope is not right for Ned's POV. As we see it, the window is clearly left of the door, and so his POV should show the twine coming from his right.