Johnnie has only fired nine shots at his wooden target but, in the flashback, he is seen firing a tenth shot in the left part of the target.
While analyzing blood spatter, Warrick and Sara have an electronic device that calculates where the blood came from for them. However, the device they are using says, "Blood Splatter" on it rather than "Blood Spatter". This is a common mistake made often by those who aren't familiar with criminal investigation.
The room the boy is found dead in is sheetrocked. As the workers are still putting up the wooden frame of the building they would be nowhere near the sheetrocking phase.