So is there anything you hate more than the old "Unknown Error" when you're doing Sharepoint development? I found this tip on Madhur Ahuja's old blog (he moved it earlier this year) and it's short enough I can paste it here:

Find the web.config for the site you normally use as your development site. Locate this tag:

<SafeMode ... CallStack="false">

and change it to CallStack="true".

Set <CUSTOMERRORS mode="On">to mode="Off".

Set <compilation debug="false" batch="false">to <compilation debug="true" batch="true">

Now you will get the full stack trace as soon as the error is raised.

Works like a charm! Well done Madhur, and thankyou!

Kate