While I was in Mexico, a restaurant added a 5% surcharge to my bill for using my US MC. This is in clear violation of the MC merchants agreement. (Also, merchants cannot require a minimum purchase for MC use. )

I called the US MC customer service line and spoke to two agents and two supervisors because I wanted to dispute the charge. Amazingly, none of them knew the rules above. In fact, they came off cocksure when telling me that merchants can set their own policies.

I still filed the dispute, making sure to include a link to the MC site that explains these policies.