This happened to us in week one prior to the addition of the panels.
The panels probably actually help to minimize this problem since without it the round bars made a "pocket" for the ball to rest in.
http://youtu.be/mdoWoOvaLAA?t=9s
The video doesn't show the ball coming to a rest up there. I should also note that the volunteer nearly knocked the ball back through the goal onto the field trying to remove it.