(Edited: 12/30/05 - New build of 8.1 20051214_2200 fixes this bug)
A little nasty bug lurks waiting to nip at you. If you edit a family's properties by first changing a type parameter and then immediately change an instance parameter. Revit will give you this unpleasant message.
Give an example? Okay, take a door family, you add an instance parameter. It doesn't even have to do anything, it can be text or length...doesn't matter. What matters is, in a project, that you change a type parameter and then change an instance parameter in the same property editing session. It only affects instance parameters that you add to the family, not those that are built in.
Fortunately the fix is like going to the doctor, telling him/her that it hurts when I do "this". They reply, "Don't do that!" If you change one or the other, close the editing session and then edit the family a second time, all is well.
Seems to affect hosted families as opposed to furniture for example. I was able to reproduce it in doors and windows but not a desk.
Yes this is true for the latest build (8.1 - 20051017_2000) as well. I've reproduced it as early as release 7.0. I don't have earlier versions installed to test, not that it really matters at this point.