An accidental side effect of early training becomes a permanent part of a behavior chain. Discussion at http://eileenanddogs.com/2012/09/12/superstition-aint-the-way/ . Before someone points it out, I know my body language is different between the time when I am on her left vs. on her right, and I am holding the treat differently. But her different behaviors are consistently connected only to which side I am on.