Being a user-interface nerd means you can’t stop critiquing everyday objects like signs and doors. Most of the time, the quirks you notice will only waste people’s time, but those that cost money deserve extra attention.

Last week’s trip to Lisbon for the IFA Global Press Conference offered a fine example of the second kind: an ATM that offered to price my withdrawal in dollars instead of euros. Its screen helpfully listed the exact price I’d pay to take out €50: $58.10. The only possible answer to that: nope!

The ATM was offering what’s called “dynamic currency conversion”–best understood, in UI-nerd terms, as a “dark pattern” set up to part the uninformed from their money. This offer amounts to an invitation to pay a premium for knowing upfront exactly how much you paid for that transaction, and you should always decline it. Even if you’re paying with a credit-card that would charge a foreign currency conversion fee.

I pressed the button next to “Accept Without Conversion,” and when I checked my bank account a few days later I saw that my withdrawal amounted to $54.22. My $3.88 in savings isn’t much, but it does represent an exceptional rate of return for a few seconds of work.

If only I’d always been that smart: Two years ago, jet lag caused me to lose situational awareness while buying a transit pass in Dublin’s airport, so I unthinkingly tapped the button to run the transaction in dollars instead of euros. I can only hope Transport For Ireland appreciated my generous donation of a dollar or two.