1. Bring up old fights. Fights that happened in the past should have been settled already, no matter how much pain it caused either of you. Keeping count of your partner's wrongs isn't healthy since it doesn't make any of you grow and move on from the hurt.
2. Have fake or insincere apologies. There's no "I'm sorry but you…"; it's just "I'm sorry" plain and simple. Some people say their sorries only to bring up something their partner did, and that's a trick that people are better off without. It leads to the blame game or something new to fight about (which is just as bad). If you want to tell your partner they did something that hurt you, confront them directly. Talking things out will make you two understand one another better.
3. Blame their partner for something bad that happened. You don't just tell your partner that they're careless or irresponsible and that's why they're not getting a promotion. That's insulting. Couples in healthy relationships are positive. They give points of improvement, they encourage their partner to be better, and they believe in their partner.
4. Hold anything about their partner's life against them. Your partner tells you their issues and stories in confidence—that you'll get where they're coming from and know more about them. It's ridiculous to say things like, "You're a nagger just like your mum" or "No wonder you don't have a lot of friends." Just. NO.
5. Post negative things about each other online. This is most likely one of those posts about not being treated right. No matter how much they seem to deserve public humiliation, you just don't shame your partner in front of all your friends, acquaintances, and officemates. That's pretty rude and it actually says more about you than it does about them.
If you're frustrated with your partner, express it to them so they can make things better. If you can't stand being with them anymore, break up.
6. Rant to other people about their partner. It's okay to vent to your best friends about something they did that hurt you, or to seek advice about dealing with a fight you had. But when you bash your partner and their behaviour to people, you're in a way tainting their image or rep, too.
7. Make their lives all about their partner. A good relationship is founded on two independent people who love each other. What's healthy is to build your lives together, since that includes talking about your dreams and compromising on some choices and come to an agreement. But to make your whole life revolve around a person who can just leave you? That's not healthy.
8. Withhold apologies. Some people just don't like admitting they're wrong. It could be pride, it could be shame. In any case, the one who's in the wrong should be able to say sorry. It might not exactly fix things, but it does console the injured party—and that's the start to working things out.
9. Withhold forgiveness. If you're pretty mean, it would feel glorious when your partner looks so vulnerable when they're asking for forgiveness and you don't see that very often. But keeping up with your pride isn't going to do you both and your relationship any good. None of you should be using the other to feed your ego. Couples in a healthy relationship couldn't bear intentionally making their partner feel like crap because they truly love and respect one another.