worst case was the corrupting of firmware for a specific network card, leaving the computer with no network.
http://lwn.net/Articles/300202/
there was a workaround fairly quickly, and the true cause was eventually found
http://lwn.net/Articles/303390/
and it was possible to re-upload the firmware in most cases. i think only a very small number of people were effected, you won't find more than a couple of angry rants (compared to hundreds when ubuntu moved the window buttons).
the odds of this kind of thing are very small.
a more likely, but still rare possibility is filesystem corruption bug. as long as you have a backup you should be fine.