If some unexpected error is thrown during build processing, then the build dies and all recipes are aborted. However, the recipe controllers do not in this case send out a collection completed event, which is how the agent status manager is notified that an agent is now free. The agent stays in the building state. Eventually the status manager will decide that an idle ping is unexpected, but then it sends out a recipe error event expecting this to stimulate a later recipe collection completed event. As the recipe controller is long gone, this latter event never arrives and the agent is stuck until restarted.