Great Prop in a Show: Chalk

I love watching shows and movies where someone has bothered to pay attention to a small detail. 

In Elementary, the police station conference room has a chalk board. Whenever people meet in the conference room, different things are drawn on the chalkboard. Most of the time, it is blank (for obvious continuity reasons) but it is rarely entirely blank. 

Many times, it has a note in the bottom left hand corner: Do not take chalk. 

I love it!