Take a look at this video:
http://www.youtube.com/watch?v=HBwPsmAPN5M
The two robots on the coopertition bridge are balanced before the buzzer, but with about 5 seconds left, at least one ball becomes wedged under the bridge.
Per [G14] Strategies that use Basketballs to either aid or inhibit Balancing of any Bridge are not allowed.
Violation: Technical-Foul, and counting or discounting the affected Bridge as Balanced, as appropriate.
I believe that, in the case shown in the video, the bridge was already balanced before the ball entered, so the bridge would score like normal.
But, if the ball entered before it was balanced, and the robots took advantage of the presence of the ball to obtain a balanced bridge, the penalty would apply.
Watching scrimmage video, I see plenty of examples of balls under bridges not coming out like they are intended. I foresee lots of controversy when bridge balancing is achieved when there is a ball stuck underneath. If a ball is stuck under a bridge before balancing is achieved, is it possible to balance the bridge without benefiting from the stuck ball in some way?