I've not heard of using WD-40, nor have I experienced that at a crime scene.

Thanks to shows like CSI, the general population has a false sense of what forensics can do, what their local police department can do and more importantly, the bad guys try to do things to thwart the efforts of law enforcement based on these assumptions.

As a patrol officer, I used to get a kick out of the drunks having pennies in their mouths when asked to provide a sample of breath. The urban legend was, the copper would lower the alcohol level reading. The more funny thing was, they wouldn't use copper pennies, but ones minted after 1984, which are zinc. Got to laugh at them twice.