Comment by NimadFlow

The most surprising indicator of messiness I encountered wasn't in the code itself, but in the tests. I had two tests that looked like tests but didn't actually measure anything—they simply returned True — and neither static analysis nor coverage tools caught them. The only thing that detected them was mutation testing—deliberately removing a line of code to see if the test fails; without that step, the test is essentially toothless. Also, I found that scores became inflated if you counted cases where a test passed despite the removal of code (because another test caught the issue) as a successful "detection."