Symfony displays a nice screen whenever a problem occurs in your code like when an exception is thrown or a PHP warning is triggered. This eases the debugging of problems as the screen gives you the issue, the location of the error, and the full stack trace. And if you setup the ide option, you can even click on any file to open it in your regular IDE or text editor at the right line. That's really nice.
But as of Symfony 2.2, this feature is even nicer as fatal errors are also nicely displayed in your browser like any other errors.
Here is how a fatal error is displayed in Symfony 2.0 and 2.1:
And here is the same fatal error displayed from a Symfony 2.2 application: