Super Select glitch on node edit

If I edit an existing node, I get the following error messages upon saving:
Only one selection allowed for
required.

This happens even though the node already has one and only one radio button selected for that particular taxonomy term. I then have to "select none" and type the term in the text box that's ordinarily used for new terms.

This doesn't happen all the time, but seems especially prone to happen when I'm editing the author or publishing information.