Poland could respond with a couple of missiles in the Kaliningrad Oblast hitting fairly non-descript targets - say, a couple of barns. The response should be proportional and immediate.
I am fairly sure that Putin did not intend to drop them in Poland, although you can never tell. If it was an accident, then the Russians will probably just accept the retaliation and that will be that.
Hmmm. For an S-300 to land in that area of eastern Poland it would need to be fired from Belarus, a bit south of Brest.
It was actually anti-missile fragment, not Russian missile.