Unable to log to session: unity problem

I was surprised after I've updated my Ubuntu 12.04 finding with the following error
"Failed to load session "ubuntu-2d" when trying to log into my laptop. I checked the log in .xsession-errors and there was a warning: xsession-errors unable to find default provider metacity
Let me tell you how I solve it.First I run the following:

sudo apt-get clean

sudo apt-get install --reinstall metacity

sudo shutdown -r now

After rebooting my desktop look a little tricky without unity. So I did

unity --reset

but result in No command found :S, so I installed with

sudo apt-get install unity

and then reboot my laptop for trying again. It worked! :)

My lesson was: if you're in trouble with a process first try to reinit it, if anything else works then try to reinstall it.

Did you have problem trying to boot with Unity? Would you like to share it?