> the certificates manually created a long time ago (predating any standardized monitoring systems) with long expiry dates Like the ever-popular "expires in 10

> the certificates manually created a long time ago (predating any standardized monitoring systems) with long expiry dates Like the ever-popular "expires in 10 years" long-lived certificates. I've seen that happen: the VPN certificate, probably created by one of the founders 10 years ago when the company was tiny, expired one day without warning, breaking the VPN for all employees until it could be replaced (manually on every device).