Before everyone gets hysterical about "starting WWIII":
I doubt that this stray missile (if that's what it was - certainly, it's in Russia's best interest to claim that it was, even though Lviv, Ukraine's westernmost city, is over 40 miles away) will actually cause WWIII or a thermonuclear war.
That said, I do expect Russia will get a "time out" for this. Whatever that looks like.
NATO and the collective West can still do plenty to retaliate against Russia for this, short of initiating war. More economic sanctions, more military/economic aid to Ukraine, more actions against the hundreds of billions of dollars of Russian assets that are currently frozen.
Such reckless military sloppiness also undermines Russia's relationship with its few remaining partners around the world. Xi Jinping might be broadly aligned with Putin against the West, but does he want Putin to be a loose cannon raising the risks of WWIII? No, not really. Expect to see some Chinese strong-arming of Russia, especially if something like this happens again.