I've had the bike take off on it's own twice so far. Both times it was because of an open in the throttle ground circuit.
It's that damn Hal sensor circuitry !!
I built a "proper" throttle using a potentiometer with a resistor before and after the potentiometer.
When I disconnected the ground from the circuit, I registered a throttle fault because +5 V was sent down the signal wire.
With a Hal sensor throttle +4.1 V is sent down the signal wire with the ground disconnected.
I don't know how to build it, but I Know it's possible to redesign the Hal sensor circuit to go to +5 V when ground is lost.
A mid-drive throttle (with the connector plugs reversed) puts out +4.1 V at zero throttle, and +1.2 V at full throttle, so when it loses ground, the throttle goes to zero, or registers a fault and shuts down.
Being as the resistors before and after the potentiometer are roughly the same, simply swapping the positive and negative going to the throttle, switches the throttle from mid-drive to hub-drive, or vice versa.