An iPhone application identifies and counts all purchases of a particular product from a shopping...

The application failed because of a defect in the purchase identification module caused by a programmer mistake or an error in the specification. A defect is a flaw or imperfection in a software product that causes it to fail to meet its requirements or expectations. A failure is an event or behavior of a software product that deviates from its requirements or expectations. An error is a human action or decision that produces a defect. A mistake is an incorrect action or decision that leads to an error. In this case, the application failed because it incorrectly counted purchase attempts, which was a defect in the purchase identification module. The defect was caused by either a programmer mistake (such as writing wrong code) or an error in the specification (such as defining wrong requirements). A detailed explanation of defects, failures, errors, and mistakes can be found in [A Study Guide to the ISTQB® Foundation Level 2018 Syllabus], pages 5-6.