JQuery sortable list - attempting to cancel a drag throws strange errors

JQuery sortable list - attempting to cancel a drag throws strange errors

I'm trying to create a screen where you can only drag three items from one list to another.
I'd like to prevent the use starting another drag if three items already exist, but the control gives very peculiar behaviour.
I'm getting a jquery-ui outerWidth exception thrown.
Can anyone help me out by explaining where I'm going wrong?


Thanks, 
  Jason