The top teeth are always supposed to be slightly overlapping the bottom teeth. If the teeth butt together or meet head on, that's a case 1 malocclusion and the easiest to fix. If misaligned teeth go too long without being taken care of, the rabbit cannot eat or even open its mouth and the teeth might start growing into the skull and cause brain damage. It's rare but it has happened. This is why it's important to regularly check your rabbit's teeth for any signs of problems.
Sometimes a rabbit can pull its teeth out of whack by biting on the metal wire of its cage. Other times, fighting with a new rabbit can also cause this problem, as rabbits commonly use their teeth when they fight.
Once teeth get out of hand, there isn't really a way to fix them. You can clip the teeth, but usually if they need to be clipped once, they will need to start being clipped monthly and the teeth will never be the same again. If you notice you're rabbit's teeth aren't the way they were before, have a rabbit-savvy vet show you how to clip the teeth. It isn't too hard and doesn't cause the rabbit pain.
How can this problem be avoided?
Providing our rabbits with hay daily is necessary for not only its digestive system but for its front and back teeth as well. We can't see the back teeth; they only way to tell if they are doing badly is by the rabbit drooling. If the back teeth get out of hand, the rabbit needs to be put under anesthetic to get them back to normal.
Hay and untreated wood (not ceder or pine, fruit branches work well), for the rabbit to chew, as well as pine cones and cardboard, are easy, harmless toys that rabbits enjoy to play with and also keeps the teeth clean and worn down properly. It's like little toothbrushes that parents buy for their baby to play with, suck on and chew. While the baby is having fun, it's also getting its gums and teeth cleaned. :)