While in development, the friendly error page that SharePoint displays when your page has an error can make debugging your master page issues very difficult. You can make an easy change to your Web config file for the site to turn friendly error messages off, therefore providing you with more useful information if your page breaks. Only do the following in a development environment, you would never want to turn off friendly error messages in a production environment. Also be sure to make a back up of the file prior to editing.
- On the Web server, navigate to the site directory:
Local Drive:\Inetpub\wwwroot\wss\VirtualDirectories\[directory for site] (for example 84) - Open Web.config in Notepad.
- Search for "CallStack". Change the CallStack status to "true".
<SafeMode MaxControls="200" CallStack="true" DirectFileDependencies="10" TotalFileDependencies="50" AllowPageLevelTrace="false"> - Search for "CustomErrors". Change the mode to "off".
<customErrors mode="Off" /> - Save and close the file.
来自下面的:http://www.heathersolomon.com/blog/articles/BaseMasterPages.aspx