On occasion, when jogging Z downwards with the Tormach shuttle ring (no arrow keys involved) the Z axis will reverse direction when I release the ring. Speed of jogging doesn't matter, and I can twiddle the ring back and forth without effect (which also confirms that it isn't a sticking ring). Only hitting the [esc] key stops the motion. Fortunately the unintended Z motion is upwards. Subsequent jogging is normal, ring and dial both.
Edit:Since I bought the Tormach shuttle I don't use the keyboard at all for jogging. The shuttle is too convenient.