…y not correct. No amount of human diligence or testing is sufficient to prevent all possible bugs. Even organizations like Google or NASA make programming mistakes, despite the extreme rigor they apply to their most critical code.
Blaming mistakes on individuals is pointless, because all programmers, no matter how experienced, have a nonzero likelihood of making a mistake. Instead, the purpose of a postmortem is to identify what in the process allowed that mistake to get deployed.