It's not common, but it absolutely happens: https://www.courthousenews.com/man-behind-s-f-system-lockout...

Early in my career in the mid 2000s, the startup that was on the same floor as mine laid off a QA person, who then showed up the next day and fatally shot the CEO and head of HR. Our CEO called me and told me not to come in that day.

This[0] is just the first result searching and it is from this week too. But it is not uncommon. Insider threats to infrastructure exist at all times of course but a disgruntled employee with administrative access and knowledge of the infrastructure can do a lot of damage quickly.

[0]: https://flaglerlive.com/it-attack-firing/

A single person can cause a lot of damage.