Poland is a member of NATO, which means that all of the other NATO members, including the United States, have agreed to intervene if Russia attacks Poland. Luckily, this has not happened yet, but the possibility of a Russian strike into Poland opens the door to the scale of conventional conflict that we have not seen since the end of the Second World War. Luckily, NATO has not yet openly intervened, though most NATO members are donating arms to Ukraine.
I think this is for two key reasons. First, I suspect that the strike was an accident on behalf of Ukraine. This makes it a tragedy on a small scale, but not something that would cause a more general war. Sadly, collateral damage happens in conflicts.
Secondly, even if the missile was Russian, Russia would have immediately apologized to Poland and offered some reparations. Current Russian conventional forces are losing, in no uncertain terms, to Ukrainian military and paramilitary forces. If Poland were to get involved, let alone Germany or other NATO allies, the entire conventional Russian military would likely be ground to fine powder within a few weeks.
All-in-all, I think this incident was an accidental one, but these kinds of accidents or small-scale events can lead to much larger effects. This is yet another lesson in the horrors of war and why it should be generally avoided.