Ellen Burstyn and William Friedkin
Burstyn was in the movie The Exorcist and reportedly had concerns about one scene, where she was pulled back by a stuntman. Friedkin secretly instructed the stuntman to pull harder, resulting in Burstyn landing on her tailbone and really hurting herself.
Rihanna and Chris Brown
These two were clearly a match not made in heaven. Any relationship involving domestic violence is doomed to fail.
Katherine Heigl and Judd Apatow
Heigl apparently was not happy with the script in Knocked Up and verbalized her feelings many times.