This time only one active engine was running and at only 4 mph. Other times there may have been several engines active and moving but I don't believe it was always the active engine that I was operating on the remote that ran out of control.
This has happened with the old hand held remote and the new wifi and iphone app. While I don't think it was me hitting the speedometer around 120, your suggestion is good practice and I will set limits on each engine speed.
Also, the 'E' button did nothing on the app with this last occurrence.