Equivalent mutants, and why the score is never 100

Some mutations produce code that behaves identically to the original, so no test can possibly kill them — and they are indistinguishable from real findings without reading each one.

// a mutant that cannot be killed: the loop is bounded elsewhere
for ($i = 0; $i < $count; $i++)      // original
for ($i = 0; $i <= $count - 1; $i++)  // mutant. identical.

// and the suppression, which is a claim worth reviewing
/**
 * @infection-ignore-all
 */

A target of 100% is therefore wrong and chasing it produces suppressions rather than tests. Somewhere between 60 and 80 is a defensible target for a mature codebase, and the number matters less than the direction. Each suppression is a claim that a mutant is equivalent, which is occasionally wrong — reviewing them like any other assertion is the discipline that keeps the tool honest.