The problem

Lucas orders the last pair of boots in size 43. The payment service charges his card for 120 euros. Then the stock service answers: someone else got the boots two seconds earlier.

Each service has its own database, so one SQL transaction cannot cover all three. Support refunds Lucas by hand the next morning. That week, the team counts 400 orders that ended half done.

The rule from the business: every order ends either paid and shipped, or cancelled with no money taken and the stock back on the shelf.