Worst mistake you've made/seen in Cyber? Ramifications?

So I just recommended a client to block the legitimate rundll32.exe. Lasted for 6 hours before I noticed and unblocked it. 6 business hours. 3 malicious .dlls all had the same hash, but different names. I got caught slipping after a 8 hour investigation, seeing the parent process rundll32's hash right above the backdoor .dll. Worst mistake I think I've ever made in my 2.5 years in Cyber.

Of course, in my first year, I also blocked all routing into …

block business caught client cyber cybersecurity hash investigation malicious mistake names process rundll32