-NK attacks SK/another SK ally.
-China invades NK (They really don't want a US controlled nation near their borders, and like stability)
-SK/ Japan/ some other US ally doesn't get the note that China is taking control of the situation, and fire on NK, accidentally hitting China's soldiers
-China returns fire,
-Conflict continues to escalate.
Unlikely, but possible.
1. I don't think South Korea would be that upset. ROK-PRC relations are actually pretty good.
2. Japan might not like it, but Japan should not be in the area to fire on anything. Venturing out from the Home Islands and being off the Korean coast/in Korean airspace would be seen as a provocative move by *everyone* in the region, including South Korea. People still haven't forgotten that whole "Japan conquering ERRYBODY UP IN HERE" back in the early 20th century.
3. IMHO, biggest potential for an accidental dustup there would be some American commander who doesn't like the Chinese. But it wouldn't be any kind of officially-sanctioned policy. Most worrisome possibility there is that some gung-ho commander intervenes and comes into conflict with Chinese troops, and then domestic political pressure from the Right prevents the President from disavowing the action and apologizing. "Saving face" isn't just a concern for the Chinese. :-/
4. In any case, I can't imagine even that sort of incident escalating into a full-on conflict unless there were numerous other factors at play (a trade war, geopolitical friction over other disputes, etc.)
Bottom line: We're not going to start a war to "save" North Korea from China. Especially when quality of life (and even
degree of freedom) for North Koreans would probably be considerably improved under Beijing's administration.