“An unexpected error has occurred” error message in SharePoint.

Here are the steps for getting the full error messages

1)Go to the MOSS site for which you would like to enable it.

Most likely to be found at c:\inetpub\wwwroot\wss\VirtualDirectories and a sub directory with your web application’s port number.

2) Locate and open the web.config file for editing.

3)Find out the following entry

<SafeMode MaxControls=“200“ CallStack=“false“ DirectFileDependencies=“10“ TotalFileDependencies=“50“ AllowPageLevelTrace=“false“>

And make following changes to it

<SafeMode MaxControls=“200“ CallStack=“true“ DirectFileDependencies=“10“ TotalFileDependencies=“50“ AllowPageLevelTrace=“true“>

4) And

<customErrors mode=“On“ />

To

<customErrors mode= “Off“ />

5)Save and close web.config.

Reference:
http://nishantrana.wordpress.com/2009/03/24/%E2%80%9Can-unexpected-error-has-occurred%E2%80%9D-error-message-in-sharepoint/