Coverage tools only tell you have tests that hit each line, but most serious bugs aren't the "you didn't hit one line", they're the result of specific interacti

Coverage tools only tell you have tests that hit each line, but most serious bugs aren't the "you didn't hit one line", they're the result of specific interactions between multiple paths. e.g. (note this is a super simple and trivial example) int size = 0; char *buffer; if (a) { buffer = new char[1]; size = 1; } else if (b) { buffer = new char[2]; size = 2; } ... if (b) { buffer[1]++; } ... Coverage tests will happily show that every line is hit, without tests necessarily hitting the buffer overflow if an and b are both true.