Other mistake: When Shane crashes the car through a fence (or wall), it appears that the car sustained absolutely no damage on the front (or anywhere else as far as I could tell). Later, there seemed to be very little damage to the car as a result of a chase scene where the car hit things, just a relatively minor scrape or two.
Other mistake: When Shane is installing the security cam, he looks to verify the image, but we are shown a standard quality image that you would find in a typical security cam. If he was on a top priority project for the military, the video quality would be way above what we see. It would also most likely not look like an off the shelf video camera (i.e. it would likely be less detectable than what he put up).
Other mistake: The mom comes home and finds the Sound Of Music poster for the performance that her son is in. Louisa on the poster is a completely different actress from the one in the rehearsal scenes.
Other mistake: When Shane is in his officer Navy uniform in the hospital, after his release from being shot, you will notice the belt buckle is centered incorrectly. A for effort, F for wrong way. Any military uniform, the belt buckle is centered off of the zipper flap - to the left, but in the movie it is centered off to the right. Another military uniform mistake. Someday these movie makers will get it right.
Answer: No, it is just his sock.
It's a cast with a sock over it. Since the movie is shot out of sequence, possibly he might have hurt it during production.