It started happening after I installed the Helvetica font on my system (at least I think that was the trigger).
I tried to install Helvetica on my Ubuntu 12.10 system but the font viewer crashed when I clicked install.
And since then all sans serif fonts are rendered like this in Chrome and Firefox:
I was able to replicate the issue in LibreOffice earlier by switching to the Helvetica font. But currently I'm not able to reproduce it anymore. I also can't find the font under the normal directories to remove or delete it.
It seems it only happens when it's trying to render the Helvetica font family, but no Arial or any others
I've also already cleared the font cache