This has happened a couple of times to me know. If I add to many OperationContract's to a ServiceContract, the WCF Test Client app throws an exception:
"Failed to add a service. Service metadata may not be accessible. Make sure your service is running and exposing metadata."
In the details it continues:
Error: Cannot obtain Metadata from . The request failed with HTTP status 400: Bad Request.
If I remove a couple of operation contracts then everything is fine. Outside of the test client is also fine.