Who fired the missile into Poland?

This week seemed like a good week to check back in with my most trusted source of information on the war in Ukraine. On Tuesday, two people were killed in Poland by what appeared to be a missile strike. A NATO member state under attack, maybe by accident? Time to hold your breath. Ukrainecast, as usual, helped me make sense of it, with expert clarification on what actually happens when a NATO country is attacked, or believes it is attacked.