One of our (now ex) developers would write tests that setup a mock, and test the properties of the mock, so if you deleted the source code, the test would still

One of our (now ex) developers would write tests that setup a mock, and test the properties of the mock, so if you deleted the source code, the test would still pass! His pull requests sucked up hours of other devs time, but bad stuff still leaked through simply because the other devs didn't have time to rewrite his PR. He was fired eventually, but the ongoing cleanup still continues...