This is posted around the web but it's easer just go to my blog than to Google it when I forget how to do it... OOB SharePoint will show some wack error message, for example:

Setting the SharePoint/SafeMode CallStack attribute in the web.config to 'true' will show a detailed error message.

Ahh, much better!

Some posts on the web say that the customErrors mode attribute (Under system.web) needs to be set to 'Off' or 'RemoteOnly' for this to work. 'Studies have shown' that the SharePoint handles its own bidness when it comes to error handling so it doesn't care what you have set in customErrors, it's totally unrelated.