Have you a WCF service and you are trying to Debug it?

Are you unable to step into the code from you test application... And is this the first time running your WCF service on your server or local dev machine?

Well if you have set the web config file correctly - <compilation debug="true'/> AND <serviceDebug includeExceptionDetailInFaults="true"/> 

then try this - Reboot your machine!

Sounds obvious but I would have liked someone to call out this most obvious fact! It would have saved me 2 or 3 hours.. I obviously had some IIS issue.. possibly iisreset may work too:-)