WCF Client Inner Exception: "The Security Support Provider Interface (SSPI) negotiation failed."

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Creating a WCF Client in Visual Studio 2010, I struggled with this error.  I was running as the currently logged on user and the service was as well so this made no sense to me.  I found that this entry in the client app.config was the culprit:

<identity>
<userPrincipalName value="MYDOMAIN\MyUsername" />
</identity>

Simply deleting these 3 lines allowed me to test the client.

Let me know if this helped you!