[via Angus]

Renaud Comte has a great tip on how to get the full ASP.NET errors appearing in WSS 3.0.

The detail is to add the following item to your Web.Config:

<SafeMode MaxControls="200" CallStack="false" ...> <!-- Becomes -->
<SafeMode MaxControls="200" CallStack="true" ...>

<customErrors mode="On" /> <!-- Becomes -->
<customErrors mode="Off" />

<compilation batch="false" debug="false"> <!-- Becomes -->
<compilation batch="true" debug="true">

Check Renaud's blog post here.