Quoted from EalaDubhSidhe:
The ball against the rubber could sit on the centre point of the shaft, but the only time such a trap would be conceivable during play is if the pop failed to fire. Bumper skirts when set up properly are raised above the playfield floor, and the trigger radius of the bumper is wider than the shaft in the centre. The skirt is also spring mounted and has its own push against the ball. There's not to say that such a trap can't occur in any game, it's not actually difficult to set a ball against a pop on any old game that's switched off and have it sit there in equillibrium.
In the second example, the trap can't happen unless the gate was to be physically opened and the ball stuck under it. Again, you couldn't guarantee that it wouldn't ever happen, but a simple nudge would be sufficient to release the ball for either trap if it actually did.
Fair enough...just the engineer in me thinking thinking outloud
I remember seeing some PF CAD drawings once that showed the design teams comments about potential PF problems...always stuck w/me.