I spend a lot of time writing quality gates to enforce invariants in my software. For weeks, my test runner stayed green. Every guard was in place, every check passed, and every test suite reported clean execution.
Then I asked an uncomfortable question: are these gates actually measuring anything, or are they just executing without complaining?
To find out, I wrote a script to sabotage my own engine. The tool parses the source code, finds every fail() assertion call in the engine, disables them one by one, and re-runs the entire mutation test suite for each line. It asks a simple question: if I delete this guard line, does any test notice?
If no test notices, that line is uncovered. It isn't a protection; it's a comment pretending to be a guard.
I expected to find a few gaps. I even wrote that expectation directly into the script's exit codes, annotating exit code 1 as (expected in v2.4.1). I knew I had blind spots—I just didn't know how many.
