Meteor 1.3-rc.12 - getting errors without source maps

There’s something wrong with my project, I’m pretty sure, and not Meteor. If I start a new project, it works fine, but in this old project I have that I upgraded to RC12, this is happening:

Is there something I need to reset in this project? Are there some old packages hanging around causing problems?

Not knowing what you have in the project. I had cleared the .meteor directory [rm -rf ~/.meteor] before doing the update. But I’ve also removed all usage of the mixin and use React Komposer instead. Not sure if this helps.