when I went to Vegas for VNEA and BCA tournaments, I "found" enough slightly used chalk to last until the following year. Figured I'd spent a grand or so attending, might as well have a few take home gifts.
I don't steal chalk, but I always take a fresh piece from the local tournament sign-up table and keep it with me for all my matches. Half the time the chalk left on the tables is worn down to the point where all you're chalking is your ferrule. At the end of the final match I'll leave the good chalk on the table and throw the worn out piece into the trash.