Not2EXceL

So I was following this tut so I can edit a base class. http://www.minecraftforum.net/topic/1854988-tutorial-162-changing-vanilla-without-editing-base-classes-coremods-and-events-very-advanced/ And everything works, except when I launch I get a ClassCastException on my DummyModContainer subclass. eg. "public class EchoDummyModContainer extends DummyModContainer" So I nop'd it from the loadingPlugin subclass and everything works. Any idea why? or why it would be bad to do this? And since I couldn't find anything else on the ModContainer, what's its purpose other than back when there was corecods.