Imported model gives error: "Transform.rotation is no longer valid"

I have a Blender model that I'm importing into Unity. It has a Bone that is giving me the error: "transform.rotation of X is no longer valid. Input rotation is [...]".

I don't see anything obviously wrong with the bone. I tried changing rotations to/from Quaternions on the Armature, no effect. Of course I have cleared all rotations, as well as recalculating bone role angles.

I have found a few references to invalid rotations on the forums. One answer suggested that the level of precision for the Quaternions wasn't high enough - but that was for a 3D motion tracker. :) I'm sure Blender has enough digits. Another answer... well, there was no answer - one poster sent the (broken) model to another person for a solution, and if they did find a solution, it wasn't posted to the thread. Which is the advantage of Answers, and why I'm posting the problem here - so the solution will be publicly available. Assuming, you know, someone does have a solution. :)

One thing that just occurred to me - is this actually a problem, or Unity fixing a problem? The second part of the error message is, "Input rotation is [...]" and some numbers that are very close to zero (or 1, like 1.0000002). Does this mean that Unity modified, as in normalized the rotation? Assuming it's close enough to the original Bone rotation, would my animations work correctly?

I usually try to fix warning messages in my code, on general principle - much less error messages. But if this message indicates that Unity applied a one-time fix to an incoming model, then I can live with it. :)

Have you tried baking the rotations in Blender, and exporting the updated file as an FBX to Unity? It's generally a good habit to form, as it prevents lots of wasted time trying to get models to rotate correctly. You can bake the rotation of a model by pressing space-bar, Transform, Clear/Apply, Apply Scale/Rotation to ObData.

I'm answering this just to close the Question. I suspect it's just a glitchy warning message that I can ignore.