When debugging mixins, it's useful to be able to see the finalized classes with your changes and injects inserted. Mixin provides a flag that allows for this:

-Dmixin.debug.export=true

This should be placed in your VM options. Once your classes have been loaded, they will appear in \run\.mixin.out

Dumping every mixined class can be useful, but is often unnecessary and does slow down minecraft a bit. Mixin provides a convenient annotation for applying debug features to individual mixins:

@Debug(export = true) // Enables exporting for the targets of this mixin
@Mixin(...)
public class MyMixin {
    // Mixin code here
}

Some classes may not appear until the game is running (or a world is loading).