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.
> 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.
> 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
This happened when a Ukraine's missile hit Poland by mistake and killed I think 2 people.
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))
