Anyone who's done any bit of SharePoint development is probably familiar with the completely useless "Unknown Error" view. Well, in fact, SharePoint actually knows what the error is, it just doesn't want to tell you (okay, it's really just disabled for users).
In ASP.NET, you can usually get error messages to show up by setting <customErrors="Off"/>
However, this is not sufficient with SharePoint. As Stefan Keir Gordan points out, to get the nitty gritty details, you also need to set <SafeMode CallStack="True""/>