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 now purged from the source family file as well.
This is obviously by design, and when used properly by someone who is conscious of what is happening, can be a good thing. When you edit a family from a project where family types have been added or deleted using the "Edit Family" tool, the resulting family contains ONLY those types that exist in the family definition in the project - the types that you added are now part of the family - but those that you may have purged are now gone.