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At a shop I visit often, the receipt checking is random, triggered by some prediction algorithm based on your history and current cart contents when you swipe your membership card when you exit the store. You get selected once out of perhaps 20 to 50 visits.

As a bonus, you get a small candy bar or similar "sorry we bothered you" gift if the check does not find any errors.

And if there is an error?

In the UK with Tesco's:

You have to scan every item in your shopping. There is no immediate penalty, but who knows what the system does in the background. I guess you could get banned from the system.

Do you get something better if they find they have made an error?

At my Sam's, they just eyeball it. A lot of times they just seem to be counting to make sure you don't have extra items, not actually checking the contents. It only takes a few seconds.

Mainly because Sam's already had receipt checking at the exit, so it's nothing outside of their normal operating process.