It doesn't happen in stationary mains powered applications. I'd have thought it was quite possible in this instance because your esc can generate much higher Hz than domestic applications. Normally stationary AC 3 phase squirrel cage motor (which is basically what brushless motors are) do not runaway or overspeed, this is because there maximum operating speed is controlled by the 50hz supply frequency. With these esc's the Hz isn't constant and changes through acceleration, by increasing the Hz I imagine it is quite possible to spin the rotor past the point it can maintain its integrity. Usually overspeeding and motor 'runaway' is assosiated with dc shunt wound motors, this generally happens when the field windings fail which causes the rotor (in theory) to accelerate to infinity and overspeed. The other overspeeding issue is with dc series wound motors like starter motors in cars, these must have a load applied or the characteristics of the motor let it accelerate until self destruction. Luckily in rc nearly all brushed motors are parrallel wound so should not overspeed!
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