If a box has two intersecting sides wdth different widths and a border-radius greater than the width of either side, the two arcs do not join cleanly, they hit each other at their respective widths. They should merge cleanly. As a note, Firefox 3 does this.
Created attachment 21817 [details] Test Case showing the issue Test Case showing the borders hitting.
This bug requires bug 19651 to be fixed to get rid of the artifacts that will show up if this is fixed.
See bug 9197 for some work in progress on this.
*** This bug has been marked as a duplicate of 17468 ***