There don't have to be any consequences, necessarily. I don't believe in karma, but that doesn't mean that people don't get what they deserve.
If you cast a spell to harm someone, they may choose to retaliate. They may also choose to call in friends of theirs to help or they may try and harm you in a greater way than you harmed them. This could count as a consequence but it's not like the person you cast at is going to or has to cast back at you. Alternatively, the energy you conjure up for the spell could linger if you don't use all of it or use it properly etc. This could affect you.
So, while those things can happen, they don't have to.