There are several key things that can instantly terminate a relationship. One of those mistakes is to cheat on your spouse. If you cheat, your spouse will instantly lose their trust in you. It will be difficult to gain trust back.
Another mistake that a person makes in a relationship is to be violent or abusive. When this happens, the other person will be afraid and it is very rare that the abusive person will change. It takes a lot of time to get over hitting someone and making them fearful.
Finally, the last mistake that someone could make would be to overspend. It may seem like a simple mistake, but money is usually the reason most couples fight. Keeping a budget would help resolve this problem.