Protecting against runaway PID

[SuperBK]

This is in C++. What are some ways to prevent runaway of the motor when using the PID controller with an encoder when the encoder is not plugged in, it fails or the wire comes loose? The only thing I can think of is monitoring the encoder for changes over time and if the motor is driving beyond a certain threshhold and the encoder is not changing, turn off the motor. The PID controller could be applying a small amount of power to hold the arm still but in this case the encoder would probably be changing small amounts, as its not being held perfectly still.

Also, we could have a limit switch connected to the jaguar to turn it off if the arm reaches its limits. Plus safe mechanical stops.

Thanks,
Brian