Are you saying the shaft can be pushed through? Like others said, it would be very obvious if the post pushed through.

I'd guess you just mean the shaft which is only held in the post by an aluminum knurled "hat" (wrong term, but I can't think of it's proper name at the moment) which is press fit onto the shaft. The wiper arm fits onto the hat. If the hat has loosend up, the shaft will drop into the passenger compartment and you will loose the wiper and hat somewhere on the side of the road.

A possible fix is to drill and tap a hole through the wiper arm into the shaft.