One more note/guess on this. I note that the "working" version of the code moves the cars with direct function calls while "sarah's" version calls methods which then calls the functions to do the move.

The latter approach is nice in that it does allow easy generation of variations and improvements on the car move methods. (It would be nicer if the methods were associated with the arena object, not the world. You could then save out the arena object and try the methods with the code for another world.)

However, it does add more levels of hierarchy to the method call that must be retraced and "stabilized" (Dick, maybe you can say this better) when the method is stopped - a possible reason the "end abruptly" may cause the problem.