First of all, can't you just talk it out? Spells aren't the solution to every problem, you know. Secondly, I recommend a simple bad luck spell, this one is very, very harsh. So unless this person you're trying to curse murdered your family or something, don't do it, the person is most likely not worth your time and effort. And if you get it wrong, probably nothing is going to happen.
If the process of casting a spell goes wrong, the effects can range from insignificant to lethal; this does, of course, depend on the spell that was cast, and what part went wrong.
If you cast a powerful spell, the effects are usually more extreme, because of all the power in the spell that was released. With smaller spells, you often don't have anything about which to worry.
Curses are not to be taken lightly. If a spell goes wrong when you are working with darker energy the energy could poor into your body. Since most people have lighter, more positive energy this could create a energy "turf war" inside your body and end up killing you. Or you could just get the sniffles. Honestly, though, I don't think it's worth the risk. Do you?