[m1tche11d]

Basically, you want to do a few things.

1. Define what "normal" operation is.
2. Set up some way to detect whether or not you are in "normal" operation. (sensors, yay!)
3. If you are operating normally, great! If you aren't operating normally, then stop the robot.

It's really up to you to decide what your robot's operating bounds are going to be. Once you know this, then you will be able to solve this problem better. Sensors are fantastic and can be used in a variety of ways.

Generally, when we have designed fail-safes on our robot, we have gone with the same sensors:

Limit switches to detect physical movement too far in some direction.
Temp reading on the KOP gyroscope to detect parts that are too hot.
Accelerometer to determine whether or not the robot is upright.

Hope this helps!