Where the FX fee hides
Some cards charge an explicit FX fee; others bake it into the exchange-rate spread, which is harder to spot. Either way, it applies to every unit of foreign-currency spend.
When it matters most
If you spend mostly in your home currency, FX barely matters. If you travel or pay foreign subscriptions, it can exceed your cashback entirely.
How to minimise it
Prefer cards with 0% FX in your currency zone (e.g. EUR-native cards in Europe), and model foreign spend in the calculator.