I always seem to forget the exact changes needed in your web.config to get some REAL error information in your browser when debugging SharePoint (other than setting customErrors = false). Yuriy came across the full set yesterday..
1: <configuration>
2: <SharePoint>
3: <SafeMode CallStack="true" />
4: </SharePoint>
5: <system.web>
6: <customErrors mode="Off" />
7: <compilation debug="true" />
8: </system.web>
9: </configuration>
There’s also this posting which offers a variety of other approaches, but I’m yet to have much success with many of them (although I admittedly haven't spent much time trying).