If a difficult situation or negative emotion boils over, it cannot be controlled any more and people start to argue or fight.

If a liquid that is being heated boils over, it rises up and flows over the edge of the pan.

Take the milk off the heat before it boils over. 趁牛奶还没有煮沸溢出，把它从火上拿下来。

If a pan boils over, the liquid in it rises up and flows over the edge.

That saucepan is boiling over. 锅里的东西煮得溢出来了。