An uncontrolled descent or ascent of a counterweight fly system's arbor and corresponding batten, caused by mis-management of either load or counterweight. The system's rope lock should not be required to hold an imbalance of more than 75 pounds. In the event of a runaway, DO NOT attempt to stop it--take immediate cover, as far away from the lockrail and batten as possible. To quote punktech: "Runaway" both a suggestion as to what you should do and a description of the situation. For more information, see "Counterweight Questions Answered," written by Bill Sapsis, on J.R.Clancy's site.