I upgraded to precise a few days ago using 'do-release-upgrade' from the command line.
I was prompted this morning to upgrade some software and I let it run. It promptly uninstalled chrome, gimp, pidgin and a dozen other bits of software, all relying on libgtk2.0-0.
It turns out that libgtk2.0-0 is still the oneiric version. I checked my sources.list and all my ppas. I found some entries that were still pointing to oneiric sources. I changed those entries to precise and ran apt-get update. Then I removed libgtk2.0-0. I was hoping that I would then be able to run 'apt-get install libgtk2.0-0' and got this error:
However, according to http://packages.ubuntu.com/precise/libgtk2.0-0, there is a current version of libgtk2.0-0 for precise.Code:E: Package 'libgtk2.0-0' has no installation candidate
For whatever reason, my computer does not know that there is a precise version of libgtk2.0-0.
There has to be a reference to oneiric still existing somewhere on my system but I can't find it.