Yeah, I have experienced it before on some Fefora 18 installation with the qemu and libvirt it packaged. Sadlly I didn't look deep into it at that time. It appeared some time when I boot a linux guest changing its size in startup sequence, or sometime vnc client resize. I guess it is caused by vnc server side code, either in qemu or vnc library, recommend try it with latest released qemu and report it later.
