When you develop a custom channel you usually develop one class which derives from BindingElement and another which implements IChannelFactory. Both classes have this method:
In many cases you override this method to return a specific set of ISecurityCapabilities. One example use case is ClearUsernameBinding:
In runtime, Wcf verifies that the capabilities the factory claims it can support are also supported by the actual channel. In other words this must be true:
If you have override the GetProperty() method of the channel but not of the factory then you will get the error above.