New threats slip through, it will always happen. It's why user training is an important part of security for a company.

It's not a case of if there will be a security incident but when, you can only limit the likelihood and damage.

Modern day reliability and security best practices are based on planning for failures assuming they are all inevitable.

Back in the old days we would just assume everything is going to work out but that just isn't sustainable now with how complex and expansive systems have become. Basically, there are too many moving parts to account for every single possibility so people should expect systems to fail and know how to react when it happens.