It was at the Zambia-Zimbabwe border at Victoria Falls in 1999. We needed to change USD into Zimbabwe dollars and were approached by an informal money changer who ostentatiously did the calculation on his calculator in full view. We knew roughly what the exchange rate should be and confirmed that the money changer was using close to that on the calculator. However, the final result of the multiplication on the calculator was fraudulently low. I thought the result was "off" but given the relatively high exchange rate (this was pre-hyperinflation), by the time I'd done the math in my head he'd taken our USD and disappeared. Not a huge deal as we were only out by maybe tens of USD, and we had to grudgingly admit that it was a very effective scam.