This highlights something that I wish was more prevalent, Path Coverage. I'm not sure of what testing suites handle path coverage, but I know XDebug for PHP could manage it back when I was doing PHP work. Simple line coverage doesn't tell you enough of the story while path coverage should let you be sure you've tested all code paths of a unit. Mix that with input fuzzing and you should be able to develop comprehensive unit tests for critical units in your codebase. Yes, I'm aware that's just one part of a large puzzle.