Continuity mistake: When Charlie is in his office talking to Larry, while going over the potential killers and victims, he writes the word "Killers" on the board and circles it. Then, when Charlie notices that Larry has left and says "gone", the writing is different. The 'k' is wider, and most noticeably, the left side of the circle is gone, and the loop in his 's' disappears. Once Charlie goes back to working, the word changes back to how it was originally written.
Picture added & edited by ployp