Currency exchange is one of the few areas of travel budgeting where the worst options are also the most convenient ones, which is exactly why they're profitable for the businesses offering them.
Airport kiosks: the worst rates, deliberately
Airport currency exchange counters routinely offer rates 8–15% worse than the market rate, banking on the fact that arriving travelers need some cash immediately and won't shop around at 11pm after a long flight. Exchange only a small emergency amount here, if any, and get the rest from an ATM once you're settled.
"Commission-free" doesn't mean fair
Booths advertising "no commission" or "0% fee" make their money by baking a bad exchange rate into the transaction itself, since there's no separate fee to disclose. Always compare the actual rate offered against the real market rate on a currency app before accepting.
Say no to "dynamic currency conversion"
When a card machine abroad asks whether you'd like to pay in your home currency instead of the local one, decline every time. This "convenience" almost always applies a worse exchange rate than your card issuer would use — always choose to be charged in the local currency.
Takeaway: Skip airport kiosks, distrust "commission-free" signage, and always choose the local currency option when a card machine asks.