What happeneds to the teeth when they are over cleaned?

Just ask my dentist! It removes gum tissue and makes your gums recede. After the roots are exposed you can actually brush away the root part of your tooth.

While the top coat of enamel is very tough, most toothpastes are based on abrasive substances (finely ground rock...) this will, over time, remove some of the surface...

However as psangster rightly says, the place where most rot sets in is not a direct 'straight through the tooth' route but a sneaky under the tooth, into the softer dentine method, rotted from the inside....

You cannot over-clean your teeth. However you can over-brush them, and as Jim pointed out, abrade the enamel, thus opening the tooth to decay.