For years the old fellow who appears to be in charge at the Pemex station between the toll road (Cuota) and Valladolid has been pulling cons on tourists.
On my most recent trip down there he tried to pull the bill-switching con (I handed him a 200-peso note, which he not-so-magically switched into a 20-peso note so he could try to shake me down for another 200 pesos). I just wagged my finger at him and drove off.
When I first started coming down, he scammed me a couple of times. These days I'm onto his tricks (he also short-changes and restarts the pump with an earlier customer's balance on it). It used to make me angry, but people have been complaining about him for years and nothing has been done, so I assume he's an institution.