In rails, how can I find out what caused a .save() to fail, other than validation errors?

Try using the bang version save! (with an exclamation mark at the end) and inspecting the resulting error.

If @user.save (for example) returns false, then just run this to get all the errors:

@user.errors.full_messages

Check all your callbacks.

I had a problem like this where I had and "after_validate" method that was failing after I had made a bunch of changes to the model. The model was valid but the "after_validate" was returning false, so if I used model.valid it said true, but then if I saved it gave me validation errors (passed through from the after_validate callback). It was weird.

Look at the application trace and you should be able to see what line of code is raising the exception.