When Cicily and Wally go to the basement, she tries to turn on the light by turning the light switch clockwise. But when Mrs Lu turns off the light in the library, she also turns it clockwise.
Joyce faints and falls straight down when trying to open the locked door but when it's forced open, she's lying much further away so that the door doesn't hit her.
At one hour in, Wally hands Joyce a pistol and goes to another room. A few minutes later, Joyce follows him and she is not holding the pistol. After she joins him, he can be seen holding the pistol.
When the heirs heading to the house in a boat point out a swimming alligator, protective tape is visibly wrapped around its mouth..
And every other closed-mouth alligator in the movie (there is one shot of an open-mouthed gator) displays the same tape.