Toward the end of the film, while Echo (Lon Chaney) and Rosie (Mae Busch) are having their conversation in the wooded area outside the cabin, both characters are clearly casting shadows on the scenery behind them, revealing that the 'woods' are actually a painting on a canvas backdrop.
When Hector and Rosie are bringing home the Christmas tree, Matt Moore is wearing his glasses as they enter the store. Once inside, they are gone. Later, as he brings the tree into the living area, they re-appear.
The goof item below may give away important plot points.
When Rosie flees the cabin to escape the chimp attack, the car that stops for her is full of police officers, including Detective Regan, who clearly know to come to the cabin. Yet, back in the courtroom when the jurors are about to deliberate and Echo decides to confess, Detective Regan can clearly be seen sitting in the courtroom. Apparently, the scene in which the police find Rosie was originally intended to follow the courtroom scene, but was ultimately placed before it.