The biggest mistake is not ringfencing this update in a test environment before sign-off for general deployment.
The ideal would be to do canary rollouts (1%, then 5%, 10% etc.) to minimise blast radius, but I guess that's incompatible with antiviruses protecting you from 0-day exploits.
Not saying CS doesn't care enough but what may be a minor update to the team that did this and not necessary for a slow rollout is actually something that really should be supervised in that way.
Except, unknown to us, our virtualization provider had a massive infrastructural issue at exactly that moment preventing VMs from booting back up... That wasn't a fun night to failover services into the secondary DC.