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I just ran the upgrade from Oneiric Ocelot 11.10 to Precise Pangolin 12.04 using the (GUI) upgrade tool. On reboot I got a garbled screen. There could be many causes for this, but in my case it was because the installation failed silently. Luckily the first thing I tried led me to the solution:

Opened a shell window -

Ctrl+Alt+F1

ran -

apt-get update

tried -

apt-get upgrade

it gave an error, so ran -

apt-get -f install

another error, referring to a blank line in /var/lib/dpkg/status, in the Description field for xmind. I removed the blank line here, then afterwards in /var/lib/dpkg/available, then again ran -

apt-get -f install

this time it ran to completion. On reboot everything appears to be ok, and it's even kept my desktop settings (Gnome Classic, I didn't get on with Unity).

My guess is the blank line in the package description of xmind led it to fail in the first place. I'll ping them.