If it damages something or someone? If relations between the two countries are not completely wretched, usually a bunch of arguing, and small reparations [1][2].
If they are completely wretched, or there's some other concern at play, usually nothing [3]. The victim country is free to apply whatever sanctions it sees fit.
These sorts of questions are not settled in a framework of legality, as much as they are settled in a framework of power, and willingness to escalate. Escalating against a nuclear sovereign is often a bad idea.
[1] https://en.wikipedia.org/wiki/Kosmos_954
> For the recovery efforts, the Canadian government billed the Soviet Union Can$6,041,174.70 for expenses and additional compensation for future unpredicted expenses; the USSR eventually paid Can$3 million.
[2] https://en.wikipedia.org/wiki/Iran_Air_Flight_655
> As part of the settlement, even though the U.S. government did not admit legal liability or formally apologize to Iran, it agreed to pay US$61.8 million on an ex gratia basis in compensation to the families of the Iranian victims
[3] https://en.wikipedia.org/wiki/Korean_Air_Lines_Flight_007
> The Soviet government expressed regret over the loss of life, but offered no apology and did not respond to demands for compensation.
Initially Ukraine said it was Russia, and the story wound down when evidence pointed to Ukraine. I do not think there were any apology (but I am not sure, I know about that because a friend of mine was nearby the hit (at a safe distance, but still))
It might be wiser to:
1. Ignore specific occurrences to avoid giving the neighborhood narcissistic bully attention. Their ego uses this for oxygen.
2. Every few months, run preparedness drills for getting to strong shelters coinciding with testing the J-Alert system.
3. Only use the J-Alert system for a credible threat.
The result is what feels like daily (sometimes multiple times/day!) alerts.
My SO's grandmother is the only one who watches cable, but the whole house runs to turn off the tv or mute it asap when we hear it.
Exactly the opposite effect/behavior the alerts are supposed to have.
It's like when your boss marks everything urgent so nothing is.
Side note- so many good songs have been ruined for me by repeated pushes of commercials using the hook/catchy part of the song max volume for whatever amazon or pill garbage. Horns honking or surens blaring.
The news this time is that government used the J-Alert system to mass broadcast to everyone in Hokkaido to announce that, instead of coming down in the sea of Japan, it might come down on the land. NHK is reporting that it might be a new type ICBM which I guess might be the reason for the general alert this time.