On Jul 20, 2010, at 3:33 AM, Joerg Hoehne wrote: > Is there (an intended) different behavior between gui and non gui > simulations? Nope. They should be absolutely, 100% identical. So this is definitely interesting. What makes it more interesting is that I cannot immediately think of a single thing in MASON proper which could possibly cause it. We go through a lot of hoops to make sure that the model code is separate from the GUI code. Perhaps: are you using AsynchronousSteppable? Or WeakSteppable? What does your GUI code do? Does it modify the model in any way beyond simply reading it to display it? I see you've customized the Schedule, and that worries me a bit. What does ScheduleExtended do exactly? Here's another possibility: are you scheduling GUI events of any kind on the Schedule? They should not be scheduled on the Schedule but rather on the GUIState's "Minischedule". In fact, let me know what field portrayals and displays you're using in general. This is a common enough mistake that we may want break Steppable into two interfaces to make it impossible to compile with this error. Sean