I've noticed that, at least in FF 3.6.13 (but this has been around a while), that the user can "click" on one of the buttons in a buttonset based on radio buttons and select the button in the buttonset but not effect the underlying radio buttons state - in other words, the visual state of the buttonset is out of sync with which radio button is "checked". I wrote "click" in quotes because it requires a sort of sliding click, ie. mousedown, move the mouse a pixel or two, mouseup.
I have a demo here:
http://jsbin.com/oribu3/3
I realize it's sort of minor, but it's easier to do this accidentally than you would think, particularly if you are repeatedly performing this as part of another process.
I don't seem to be able to submit a defect, but in searching the defect database, I couldn't find an open defect that matched this. For now, my workaround is to bind to the mouseup on the buttonset and refresh the state.