Thanks, everyone for your feedback. I'm pretty sure the problem only occurs after I make an adjustment... but I am sure that I did not decrement any zone below zero. I suspect that occasionally when I make an adjustment, the controller is erroneously reading my keystrokes to decrement the watering time of a zone that is not currently selected or displayed. When I posted my original inquiry, I was hoping to get a response that this was a known bug and here's the solution. Unfortunately, this does not seem to be the case, so the problem remains a mystery for now. I have cycled the power and reset the controller. If the problem persists, I will try to find a replacement front panel. Does anyone know a good source for older rainbird controller front panels? The model is ESP-LX+. Thanks again for all the help.