When you comment on a Facebook post because you don't like what was said, and Facebook's algorithm thinks your interested in seeing more of that topic - so inundates you with posts about it.
Facebook Karma hits you in the face when you type a comment "I definitely don't agree" with regards to a post on gay marriage rights. Facebook sees your posts and starts showing you more posts about gay marriage.
Revisiting the sins of your ex on your current relationship.
My poor husband. Because of my relationship PTSD I expect him to act just the same way my ex did.