Lately I experienced a problem with KVM-based virtual machine running a DHCP server and another one running a DHCP relay (for both I use the ISC implementations). The DHCP relay was complaining about "bad udp checksums". Using tcpdump and wireshark I quickly found out, that the software was right and the UDP checksums were in fact wrong. After some searching, I found a bug report, that basically described the same problem.
Although I cannot verify this, I think the problem might be related to the fact that I recently upgraded the host machine from Ubuntu 10.04 LTS (Lucid) to Ubuntu 12.04 LTS (Precise). As a workaround, I deactivated the use of the "virtio" support for the network interface in both virtual machines, which seems to fix the problem, because then the UDP checksums are correct.
However, when I performed the same change for a virtual machine still running on an Ubuntu 10.04 LTS host, this actually caused a problem: If VLAN interfaces are used inside the virtual machine, the normal non-virtio driver will screw things up on a Ubuntu 10.04 LTS host.
Long story short: For virtual machines running on a Ubuntu 10.04 LTS host you should use and for a Ubuntu 12.04 LTS you should avoid virtio networking.
Update [2012-07-08]: It seems like the conclusions I draw in this article are actually wrong. Therefore I posted an update clarifying the situation.
Some time ago Ubuntu 10.04 LTS received a Firefox update from the 3.6 branch to the more recent versions (10.0+).
After that upgrade, Firefox on one of my Ubuntu systems suddenly appeared in English instead of the correct locale (German in my case). I first thought that this might be a problem with some localization packages not being installed correctly. However, the problem persisted after upgrading the system to Ubuntu 12.04 LTS.
Some globally extensions (in particular the language pack) showed up in an old version in the add-ons list, although the newest version was installed. Finally I found out, that Firefox looks for the extensions in /usr/lib/firefox/extensions. The new language packs however had been installed in /usr/lib/firefox-addons/extensions. On other systems, /usr/lib/firefox/extensions is a symbol link to /usr/lib/firefox-addons/extensions. In my case however the directory existed and contained the files from the old versions of the language packs.
For some reasons, the old language packs (which had different package names) had not been removed and thus the Firefox upgrade did not place the symbol link (because the directory was not empty). After manually removing the old versions of the language packs, deleting the directory and reinstalling Firefox, the symbol link was created automatically and suddenly the globally installed Firefox add-ons worked again.