Avos 4:18

Rabbi Shimon ben Elazar said that we should not attempt to appease a friend while he is still angry, nor attempt to console him while his deceased relative is still unburied, nor question him at the time he makes a vow, nor attempt to see him in his time of disgrace.

Avos 4:19

Shmuel HaKatan said that we should not rejoice at an enemy’s downfalls nor be glad when he stumbles. When God sees it, it might displease Him, causing Him to turn His wrath from that other person to the one gloating over his misfortune. (Shmuel HaKatan quotes two verses from Proverbs - 24:17-18 - that he was accustomed to caution people.)