Other mistake: The film is meant to be set in the US, but there are some odd lines of dialogue that seem to indicate the movie takes place in the England, such as a reference to the (very British) Julia being on her "home turf." This is the result of a change made in post-production, where producers decided to change the setting to the US to make it more marketable to American audiences. Unfortunately, there were some lines about the original English setting that (evidently) couldn't be cut around.
Audio problem: When the unnamed Asian man sells Frank the box at the beginning, watch when he says the line "it always was." The words don't quite match the lip movements, revealing the line was looped in post.
Revealing mistake: When Frank is solving the puzzle-box in the opening scene, you can pretty blatantly tell when they switch out the prop, because little lines, gaps and shadows appear on the box that weren't there before. (Since it wouldn't be possible for a single prop to do all the different "functions" of the mystical box.) It's most noticeable when the first moving part appears, and you can see several hard "lines" that weren't there in previous shots.
Answer: Letting the moviegoers know what happened to him, he was no longer human, but a piece of meat to be butchered.