Revealing mistake: When Agent Hotchner receives a text from JJ while interviewing the suspect, it is actually a screenshot. You can see Camera Roll in the upper left hand corner, "2 of 2" where JJ's name should be, and the Share/Play/Delete banner at the bottom. It is covered by Agent Hotchner quickly tapping on the screen to clear the banners.
Revealing mistake: When Hotch and Spencer are interrogating Eddie, Spencer shows him the three photos of the people he has killed. Eddie makes a gesture as if to speak and points at Spencer, at which point we see a shot of Spencer and he very obviously glances into the camera.