I added a dependency to a test helper and forgot to declare it in the dev extra. Locally it was already installed, so everything passed. In CI it wasn't, and the tests that needed it did not fail.

Source: [Dev.to](https://dev.to/juanauriti/my-ci-hadnt-run-a-single-one-of-those-tests-in-two-months-and-stayed-green-the-whole-time-n15)

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