Process Builder picklists bug
If you use PBs a lot, read this.
So I have an org that has a huge PB. No one cares. Fine. But what's interesting is they have a LOT of decision nodes. And today I realized some of the nodes referenced "Select One" as a picklist value. Which of course would never work. Except Process Builder doesn't let you select "Select One" when you save. And a Process Builder is read only when active, so that can't happen, right ?
If you delete or deactivate a picklist value referenced in a process builder, even if the PB is active, Process builder will very kindly change the criteria to "select one". While staying active.
This affects ALL versions of the Process Builder, so you can't restore from an old one unless you stored the metadata somewhere. Seeing as Flows share the same backend, I would be wary about that as well.
All that to say Salesforce will, if your admins don't take care, kindly modify read-only automations, in prod, without notification. And of course if you undelete the value or undeactivate it, it doesn't bring it back.