At a BBQ restaurant at DFW Airport. First, the sign is nasty and dirty looking. A laminated sign would do wonders for the “last impressions” for the customer.
Better yet would be a trash can with a round template that makes it impossible to throw away the tray. This is a lean concept called “poka yoke” — aka “mistake proofing” or “error proofing.”
It's really not that hard to error proof a trash can. Our recycling bins have cut outs on the top that only allow you to put paper in them. People just have to make a little effort to error proof their world somewhat.