Sometimes when you are drawing walls that are close together you may find that they automatically join. What you are really trying to do is stop the wall short but it just keeps joining up again. This could be because it is attempting to join to something higher or lower on a different level, or a multitude of other reasons. What ever the reason, you can manually override the wall join.
But this is what keeps happening. The blue grip joins to the centre of the other wall (which in most cases is what you would expect it to do).
The answer to this problem is very simple (but not so obvious to find).
The function is called 'Disallow Join'