Can't use any GUI functions

Whenever I try to use a GUI command it doesn’t work and I get an error:

MissingMethodException: GUI.Button
Boo.Lang.Runtime.DynamicDispatching.MethodDIspatcherFactory.ProduceExtensionDispatcher ()

I’ve got no clue what is causing this, any help would be appreciated, I’m kind of stuck where I am until I can start with the menus.

Yeah, I solved this shortly after posting the problem. As Loius said, if you have a script called GUI then Unity shows this error.