From the accounts given to me by the roboteers involved, the problem occurs when a large amount of transmitters are turned on, e.g. more than 10. What happens then varies, but in some cases the robot doesn't failsafe but responds erratic, changes direction, and even fires the weapon. If that is the case then there is a real risk factor.

A very simple test for this. Put a single robot on 40 MHz in the arena and drive it, and switch on 10-15 spektrums on, one at the time and see what happens. If the problem is with the Spektrums that would show it.

From what I've gathered it also happens in the link up stage. Even if other robots are switched on and waiting in the arena, as long as they stand still no power goes to the motors, so suppression issues on the motors is a unlikely candidate in this case. Interference from the ESC's could be a factor, but then that would happen with 10-15 40MHz machines just like it would with 10-15 Spektrum machines.

I think this calls for more investigation.