Ambiance, the default Ubuntu theme, has some bugs when using the classic (fallback) GNOME session in Ubuntu 12.04 and a fix is available in a branch on Launchpad, but it hasn't been merged yet, so it's not available in the official Ubuntu 12.04 repositories.
To fix this theme issue for GNOME Classic session in Ubuntu 12.04, I've uploaded the fixed "light-themes" (Ambiance and Radiance) package to the WebUpd8 themes PPA. Add the PPA and upgrade to get the fixed light-themes package using the commands below:
The package comes with fixes for both Ambiance and Radiance themes.
sudo add-apt-repository ppa:webupd8team/themes sudo apt-get update sudo apt-get upgrade
After upgrading, either change to some other theme and then to Ambiance or Radiance, or log out and log back in.
Here's how the classic GNOME session looks like before the fix:
(Take a look at the panel text colour and window list entries; also, this re-adds the Ubuntu logo to the Applications menu).