ASP.NET's Configuration System
Have you ever wondered how ASP.NET installs its fancy 'Web.config' configuration system? Well, here it is...
HttpConfigurationSystemBase configSystem = new HttpConfigurationSystem();
MethodInfo method = typeof(ConfigurationSettings).GetMethod("SetConfigurationSystem", BindingFlags.NonPublic);
method.Invoke(null, new object[] {configSystem});
It uses refection to call a private method. I didn't expect to find that!