From a thread at RFO, Aaron explains what's gone wrong...
"If someone deletes a family that is also the default option for a Family Type, with that Shared Parameter: Yes, the entire parameter gets deleted. It's terrible behavior, and its been that way for years.
In case I wasnt clear: This is a known issue, and it's easily reproducible.
And yes, the instances in your project are hella broken, now."
- Take any Family that uses a Shared Family Type Parameter, that has a default value.
- Find the family in the project browser, that is the Family and Type that's in the default value.
- Delete that family from the Project Browser.
- After you've clicked *DELETE* in the warning, go back to the original Family (the parent family with the parameter).
- For JUST the types that had that default value set, that parameter is now gone.