As mentioned before, using the constraint-tool can both help and hinder your designing process. So use with caution.

An interesting fact to keep in mind is the following: when you create a dimension line and turn on the constraint ("lock" the lock) that constraint will remain in place, even if you delete the dimension line. So when you move one end of the constraint, the other moves as well.

This happened to me with two levels that one of my colleagues locked together. A bit frustrating if you ask me.

But the easy way out is: simply create a new dimension line! When you select it, the lock will appear and be locked, showing that there is a restrained relationship between levels. Simply open the lock and things are back the way they should be.