Check engine light on after replacing starter

My mechanic just replaced my starter and now the check engine light is on.  Prior to picking up my car, he asked me if the light was on when I brought it in.  I told him absolutely NOT   He, of course, ran the code and said it was some part that I can't remember..  I say baloney...  What could he have done or not done to make the light come on?  I certainly paid him way too much just to replace the starter.