My biggest one was with my mom. I do love my mom, but she has a lot of mental problems, and can be the Queen of PA and will be extremely controlling, if you give her that option.
When I was in college, she used to call me and leave messages on my answering machine all day (this was pre-cell phone days). As I wasn't returning her calls (because I was at class, then lunch, then class, then theater, then the gym, before going home), each message would get more & more irate. She would demand that I come back to my room just to check for messages and call her to confirm that I was still alive. I refused to do this, because it would have been close to impossible, with my schedule. Not to mention the part where it was a ridiculous request.
As this escalated, she started ending her calls by threatening to disown me, because I was such a terrible daughter (side note: I hope you believe that I'm not exaggerating here. She was literally this upset that I wouldn't call her several times a day to confirm that I was still alive, and tell her about my day. Which was remarkable, because when I lived at home, she used to ignore me quite a bit.). Finally, one day, I called her bluff. I said: Fine. Disown me. If that means I don't have to listen to you screaming at my anymore, then that works for me. I'll figure money out of my own, & that'll be that. Then I hung up the phone, gently.
This was probably in late October or early November. I didn't go home for Thanksgiving (I went to see my BF's family). I didn't go home for Christmas (I stayed with friends).
Finally, around Easter, my aunt called me & said my mom was going to call me. Seems that my dad told my mom that she would either fix this or he'd divorce her. My dad is one of the most mild-mannered people in the world, and he has *never* given my mom an ultimatum before. And my mom is of a religion that really frowns upon divorce, so she buckled. Although, her buckling was calling me and asking what I wanted in my Easter basket; she never acknowledged her bad behavior.
Since then, while she does off & on have bad behavior (she's a "give her an inch, she'll take a mile" kind of person), she's never gotten as bad as she was then. Because she knows if she goes beyond a certain point, I WILL call her on it, & my dad WILL back me up.