spree/spree — pull request #4244 by cbrunsdon (2014-01-22 22:54:52+00:00)
Title: Allow halting of callbacks by returning false

Without specifying a :terminator to define_callbacks, its impossible to
halt the event from happening.

And yes, that is a string that is eval'd, as per:

http://api.rubyonrails.org/classes/ActiveSupport/Callbacks/ClassMethods.html#method-i-define_callbacks