How caps are written
A capped bonus reads like this: a multiplier, a category, a spending limit, and a period. For example, 4x on US supermarkets on up to $25,000 of spend a calendar year, then 1x.
Caps are counted on spend, not on rewards earned, and they are counted per card rather than per person. Two cardholders on the same account usually share one cap.
Some caps run monthly or quarterly instead of annually, which makes them much easier to cross without noticing.
Why crossing one is easy to miss
Nothing at the terminal changes. The purchase goes through identically, and the difference only appears when the rewards post, often weeks later.
Statements rarely say 'you have used your cap'. They show a smaller number of points than the same shop earned last month, which is easy to read as a mistake rather than a rule working as written.
This is the single most common way a carefully chosen wallet quietly stops paying what its owner thinks it pays.
What to carry after the cap
Once a category card is at its base rate, it is usually the worst card in the wallet for that category. A flat-rate card paying 2% on everything beats a 1x card in almost every case.
If you hold two cards with bonuses in the same category and different caps, spend the smaller cap first and move to the larger one afterwards.
Caps that reset on 1 January make December and January behave very differently. It is worth knowing which of your cards resets when.