So once I figured it out, there was a generous *facepalm*. Nonetheless, in case you’re struggling with the same problem, here it goes.
I had a before_validation callback on an ActiveRecord model. I was running in test mode using Shoulda and FactoryGirl, so I figured those 2 gems had something to do with it. Then came the humble pie.
I also had a validates_presence_of on the model, and it was getting called before the before_validation callback. The test wasn’t setting that attribute, so naturally I never got to the callback.
Once again it was not a cloud if improbability that caused the universe to collapse around me. It was a bone-headed error.