WCF extensions: The type 'x' registered for extension 'y' could not be loaded.

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Quick note: if you configure a custom extension in WCF, don't forget to add it as a reference to your project. If you don't configuration will run just fine, but at runtime, the following exception occurs:

System.Configuration.ConfigurationErrorsException: The type 'x' registered for extension 'y' could not be loaded. (config information, line z)

   at System.Configuration.BaseConfigurationRecord.EvaluateOne(String[] keys, SectionInput input, Boolean isTrusted, FactoryRecord factoryRecord, SectionRecord sectionRecord, Object parentResult)
   at System.Configuration.BaseConfigurationRecord.Evaluate(FactoryRecord factoryRecord, SectionRecord sectionRecord, Object parentResult, Boolean getLkg, Boolean getRuntimeObject, Object& result, Object& resultRuntimeObject)
   at System.Configuration.BaseConfigurationRecord.GetSectionRecursive(String configKey, Boolean getLkg, Boolean checkPermission, Boolean getRuntimeObject, Boolean requestIsHere, Object& result, Object& resultRuntimeObject)
   at System.Configuration.BaseConfigurationRecord.GetSectionRecursive(String configKey, Boolean getLkg, Boolean checkPermission, Boolean getRuntimeObject, Boolean requestIsHere, Object& result, Object& resultRuntimeObject)
   at System.Configuration.BaseConfigurationRecord.GetSectionRecursive(String configKey, Boolean getLkg, Boolean checkPermission, Boolean getRuntimeObject, Boolean requestIsHere, Object& result, Object& resultRuntimeObject)
   at System.Configuration.BaseConfigurationRecord.GetSection(String configKey)
   at System.Configuration.ClientConfigurationSystem.System.Configuration.Internal.IInternalConfigSystem.GetSection(String sectionName)
   at System.Configuration.ConfigurationManager.GetSection(String sectionName)
   at System.ServiceModel.Configuration.ConfigurationHelpers.UnsafeGetSectionFromConfigurationManager(String sectionPath)
   at System.ServiceModel.Configuration.ConfigurationHelpers.UnsafeGetAssociatedSection(ContextInformation evalContext, String sectionPath)
   at System.ServiceModel.Configuration.ServicesSection.UnsafeGetSection()
   at System.ServiceModel.Description.ConfigLoader.LookupService(String serviceConfigurationName)
   at System.ServiceModel.ServiceHostBase.ApplyConfiguration()
   at System.ServiceModel.ServiceHostBase.InitializeDescription(UriSchemeKeyedCollection baseAddresses)
   at System.ServiceModel.ServiceHost..ctor(Type serviceType, Uri[] baseAddresses)
   at Microsoft.Tools.SvcHost.ServiceHostHelper.CreateServiceHost(Type type, ServiceKind kind)
   at Microsoft.Tools.SvcHost.ServiceHostHelper.OpenService(ServiceInfo info)