Whether you believe in cosmic punishment for your actions or not, when you see someone who wronged you get exactly what they deserve, it's so delicious. I had an ex who cheated on me come back and complain about getting cheated on by his current girlfriend. 10/10 would recommend.
Seeing the girl who was mean to you in high school selling essential oils on Instagram (yes I am very petty, I know)
Ex-husband left me after 10 years for "a younger model". We have two kids together, he wanted to be free and spontaneous again. Got her pregnant within 3 three months and now gets to do that whole baby and toddler stage again, that he finds so incredibly exhausting.