-2th mistake: consider dual booting when trying out a new version of an operating system

-1th mistake: not using the wonderfully powerful command line tools available in GNU/Linux to make a copy of the Ubuntu 12.04 system. Some use dd or dress it up with CloneZilla, others use filesystem level tools like tar then restore grub with a live image. Point is you can always get back...

0th mistake: going out to work without a USB stick containing a known working live image. You can boot of a USB even if the hard drive dies. Plug another USB stick in for storage.

Not gloating, I've made all these mistakes at least once!