4. Robert Downey Jr. and Sarah Jessica Parker
You’d think two celebrities with six names between them would be a match made in heaven. Unfortunately, though the Firstborn co-stars fell in love on set, the relationship began to fall apart as Robert Downey unraveled due to his drug and alcohol abuse. Looking back on the seven years they spent together, RDJ has gone on to say that he regrets hurting Sarah, and that his “selfish” actions took their toll on her.