I wrote in the past how WCF defaults limit scalability but this thing (which had cost me two days of head scratching) is even worse. Consider the following scenario:

Sounds common enough now doesn't it? And it is -- unless you happen to use a service with   WebHttpBinding (e.g. if you try to develop a RESTful WCF service or want to use POX services).  When you use WebHttpBinding and try to make a call within a call you are likely to find yourself starring at a ProtocolException with a 405 error - Method not allowed. Turns out WCF finds itself confused by the Operation Context (OperationalContextScope) of the incoming request so if you want things to work properly you need to create a new one for the request

var webBinding = new WebHttpBinding();
var channel = new ChannelFactory<ImContract>(webBinding, controlUri);
channel.Endpoint.Behaviors.Add(new WebHttpBehavior());
var proxy = channel.CreateChannel();
using( OperationContextScope((IContextChannel) proxy))
{
     proxy.Dostuff()
}

I already spent the time figuring this bugger out. I hope this post will save you the trouble.