The Angle of Engagement is an angle at which the first tooth of the piston toothing comes into contact with the toothing of the sector gear. If the AOE is incorrectly adjusted, the piston guiding rails and the first piston tooth are subjected to increased wear.
Some hop-up buckings (e.g. Maple Leaf – see photo) have a longer lip. It is possible then for some combinations of buckings and hop-up chambers to have bucking's lip protrude into the hop-up chamber's BB feeding hole.