I used to get a "product replacement guarantee" with meth/heroin. Kind of like "if the guy I'm having work for me shorts you, let me know and I'll find a new pawn for my chessboard"... sort of thing.
Not a lot of people can manage to employ people for their biz in that sense if you will, and only some drugs (i.e. meth/heroin) are profitable enough to do that sort of thing.
The FEW times something like this happened (heroin) - the dude would give me exactly what I already paid for. Why? He saved most of his client list; most people would say "nah fuck that whack shit" and just find the next dealer, know what I mean? Yeah.
If I didn't get a free bag for what I was missing I wouldn't have been so loyal... but damn, if they don't fill your fries full I doubt the big fast food businesses really care to be perfectly honest.