The attendants at the Pemex gas station in Tulum (coming into town from direction Cancun) tried this on us twice, so it must be their usual scam. Watch out for this!
The attendants are very friendly and chatty. When you pay with a 500 Peso bill, they will quickly swap it for a 50 and then act as if you had given them a 50 instead of a 500 (stupid tourist ...). Remain ademant, stand your ground and they will give you your money back.