I've had many issues with unauthorized DCC in India, for instance had dinner at a 5 star hotel in India, the ITC Maurya, and when the bill came it was priced in Indian Rupees. But after my visa card was charged, the receipt to sign came back in USD, at a conversion rate of almost 5% less than the true rate. No matter how much I tried to argue at the restaurant that it should've been charged in local currency, they wouldn't listen saying its their 'policy' to process the charge accordingly for foreign CCs.

Good idea someone posted on if Amex will not cause DCC, will try that.