I had a Symfony 1.0 app and I finally decided to make the leap to 1.2. Lo and behold, this mysterious error cropped up:
Notice: Undefined variable: site in /path/to/myproject/apps/site/templates/layout.php on line 10
If you’re upgrading from 1.0, you may suddenly notice that you get these “undefined variable” errors. That’s because global variables are deprecated in layouts as of Symfony 1.1, to be replaced with slots. Generally, a wise idea. More info here: http://trac.symfony-project.org/wiki/Symfony11LayoutUpgrade