Sometimes you discover something and your first reaction is a bit of embarrassment because you didn't already know it, followed by amazement that nobody else has ever mentioned it to you before, because it's potentially a REALLY big deal.
In today's previously unknown-to-me potentially REALLY big deal department:
Did you know that you can accidentally delete multiple family types simply by opening and saving your Revit family? It's true. Here's how it happens:
1) In a Revit project, a conscientious BIM or Model manager performs a Purge Unused, purging unused Family Types, in an effort to keep file size down. This is good practice.
2) At some point in the future, from the same project, a user selects an instance of a family on which the above-mentioned purge was done, and selects "Edit Family" from the Ribbon (or double-clicks on the family instance to go to the Family Editor - who's boneheaded idea was THAT feature, anyway?).
3) The user makes a minor change to the family, then saves the file.
*Poof* - all of those family types that were purged in the project are ...