Instead of getting the "user-friendly", but frustrating-to-a-developer message "An error has occurred", use these two steps to receive the actual error when developing in SharePoint. Works for both MOSS and WSS.

1. Show the call stack
   Change
   <SafeMode MaxControls="200" CallStack="false" DirectFileDependencies="10" TotalFileDependencies="50" AllowPageLevelTrace="false">
   to
   <SafeMode MaxControls="200" CallStack="true" DirectFileDependencies="10" TotalFileDependencies="50" AllowPageLevelTrace="false">
   
2. Set custom errors to False
   Change

   <customErrors mode="On" />
   to
   <customErrors mode="Off" />