I'm on a bus about to cross a border between underdeveloped countries. As my bus arrives, I can see a crowd offering money exchange, carrying luggage services, tricycle rides. They rush in when they see a new bus coming and even more when they see me through the window. I can see the money signs in their eyes as I'm exiting the bus. They almost fight each other to be the first at offering me whatever service they sell (at a special foreigner price!).
I'm now surrounded, it's loud, they all talk over one another. Some grab me, others pull me to try to have my attention. My awareness is divided between what is happening and my pockets to be sure I'm not pickpocketed. One offers me money exchange, as it happens I need to change the remaining currencies of the country I'm about to leave. He told me his price (all this while we have half a dozen spectators). I bargain a bit, we agree on a fair price for the money exchange. He pulls out his calculator, asks me how much money I want to exchange (I already counted that in the bus, didn't want to do this surrounded by a crowd at the border). He uses his calculator and shows me the amount I should receive. I quickly feel something is wrong, the numbers don't seem to add up, but they are too complicated for a quick mental calculation (I wasn't travelling with a smartphone back then).
He senses my hesitation, and redo the equation in front of me. I ask him to do it myself. I grab his calculator and use it myself. The final amounts are the same. While I'm trying to concentrate, a bunch of people are still trying to get my attention by talking to me. One even offers me to stamp my passport for me (as if I'm gonna give you my passport!). I hardly trust him, but I trust his calculator. So we make the money exchange. I finish my border crossing, once on the other side on a bus to the next town. I pull out a pen and paper, a take the time to do the calculation. I have done the calculation multiple times (even with an online calculator later on), the amounts don't add up. There is only one explanation; his calculator was rigged! How did he do that? I would love to know. That certainly deserves the Nobel Prize for rip-offs!
The lesson of the story: I should have paid more attention in math class at school... No seriously, listen to your gut, if something feels off, trust that feeling even with something as trustworthy as calculators!