If there's a cash transaction every Wednesday between 7pm and 9pm, which always contains a specific type of bread, and a specific type of breakfast cereal, and

If there's a cash transaction every Wednesday between 7pm and 9pm, which always contains a specific type of bread, and a specific type of breakfast cereal, and a specific type of milk, and a specific type of sandwich filling, that's anonymous customer #5678. If you ever misremember the amount cash in your wallet, and, rather than just abandoning your basket at the checkout when you discover this, you pay with a card just one time, for that exact basket, when there is no cash customer for that basket on that night, they can now be pretty certain about who customer #5678 is, and can then link you to 95% of your historic purchases too. OpSec is hard, yo.