the eurochange scam
That is, unfortunately, a common game played upon obvious (or oblivious) Americans. Anecdotes and testimonies are plentiful, and go back years. It probably helps, when paying with a large denomination bill, to make a point of identifying this when it first changes hands. A particularly brazen or contemptuous waitron will likely deny to your face, upon returning with change, that you ever pointed to the "50," but I would guess that the average petty criminal in the serving class will prefer to prey upon the inattentive, as requiring less effort to lie to.
cordially,
