If you could attach a screenshot to demonstrate the problem, that would be excellent.

Re-assigning to the gtk-engines product.

Created attachment 102756 [details]
Screenshot illustrating the bug; "thick" and "dashed" are in mixed state, while "thin" is unselected, but they appear the same.

Screenshot attached; it may not be obvious, but the "Thick" and "Dashed" buttons are supposed to be in mixed-state, while the "Thin" button is unselected. 

Also, the bug was confirmed, so to say, by Calum Benson, maintainer of HIG. In fact, he told me to report it. Link (see 16:):
http://code.google.com/p/google-highly-open-participation-gnome/issues/detail?id=4#c9

Outch, the same problem probably also exist in other engines, this needs to be checked.

I don't have any idea of how we could draw the inconsistent state right now, though.

Created attachment 116587 [details]
Mockup of 'mixed' style

Had a quick play in GIMP with some ideas, this is about the best I could come up with.  (The middle button is supposed to represent the "mixed" state, of course.)

Created attachment 132075 [details] [review]
patch implementing the suggestion from Calum

Here is a simple patch that implements Calums suggestion. Andrea, do you think there is anything against commiting it?

OK, I committed the patch. If anyone dislikes this, please speak up (and suggest something better) :-)

And to think it took a mere 2 years to fix this (including 8 months with a patch ready)!