If you are developing master pages for SharePoint, and you get errors like "An unexpected error has occurred" when viewing your site with your custom master page applied, then enabling stack traces and disabling custom error pages in web.config is a great way to help you diagnose the specific problems.
You can do this by setting <customErrors mode="Off" /> in web.config and the CallStack="true" property in the <SharePoint><SafeMode> tag in web.config.)
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