This report must begin with an apology. Not only is it non-repeatable (cannot be reproduced directly), since it was also non-persistent (do or re-do what I intended and it disappeared) and easily worked around or ignored, I do not have many helpful artifacts nor do I recall just what caused it.
The following screen grab shows B1 selected. I do not recall whether the error occurred on selection or when I chose an action (such as Edit Geometry Object).
I think I saved the model after it happened a couple of times: