Cheech and Chong called it "Stunt weed".
I am willing to bet you that sometimes it is easier to use the real McKoy they can take off of any one of the crew and tell them it was stunt weed.
I have friends that work in the studios and they are stone cold stoners with weed stories from work that blow my mind.
I was watching MAD TV repeats on Comedy Central and they did a skit about weed and the stash looked all too real to me.