Ok, first of all, watch the DVD. there is a featurette about the Making of the Special f/x of harry potter 3. in there, they mention that they put in there as a joke. they say something like "I think this is the first time a CG character has pooped on-screen before"
If you have the DVD they talk about how they thought it made it realistic. When I first saw it I knew it was a joke. PLus how could that be an mistake it not a real animal.