Showing the debug trace in Umbraco

Whilst developing with Umbraco, you'll quite often come across errors in UserControls where all you'll see is a big red box with not a lot of info as to what the error is and where it occurred.

It's easy to find this info when you know how; just add umbdebugshowtrace=true to your querystring and you'll see the debug stack trace, allowing you to locate the problem.