I've been working on an Outlook Add-in, which uses a WCF client to connect to a web service. I created my service reference using the built-in Visual Studio tools, which places the service configuration in App.config. The service client worked fine for me on my Windows 7 box, but would not work properly on Vista or XP machines, which were getting an exception : Unrecognized element 'extendedProtectionPolicy'...

I looked at my client app.config, which contained an extendedProtectionPolicy element.

<extendedProtectionPolicy policyEnforcement="Never" />

I had not seen this element before, and I am not sure why this element was added to my app.config. Everything seemed to work fine on my Windows 7 machine, but not on Vista or XP.
After removing this element, the client worked again on all machines..

Apparently there are some differences with running Visual Studio on Windows 7 ;).