Belinda the GP CSI has a great post up on the rookie mistake of goofing up the GL distributions in an AP or AR transaction. Point new users to Belinda’s post: What do I call the “Rookie Mistake”?
Avoid a Dynamics GP “Rookie Mistake”
Archived post from 2010. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.