If you run into this problem using Add Selected on leaders in AutoCAD 2011 or verticals there is a solution available.
You run the ADDSELECTED command and select an existing leader object to create a new leader. After picking the first two points, you press "Enter" and AutoCAD begins continuously prompting for the next line of text, forcing you to use Task Manager to kill the AutoCAD process.
For the solution see KB: TS16158933
With the ADDSELECTED command, you can create a new object with the same object type as a selected object. It is also possible extending ADDSELECTED to support custom objects by editing the AcSelCmdMap.xml file.