From 737org.uk:

3 March 1991, UA585, a 737-200Adv crashed on approach to Colorado Springs. The aircraft departed from controlled flight approximately 1,000 feet above the ground and struck an open field. After a 21-month investigation, the Board issued a report on the crash in December 1992. In that report, the NTSB said it “could not identify conclusive evidence to explain the loss of the aircraft”, but indicated that the two most likely explanations were a malfunction of the airplane’s directional control system or an encounter with an unusually severe atmospheric disturbance.

8 Sep 1994, US427, a 737-300 was approaching Pittsburgh Runway 28R when ATC reported traffic in the area, which was confirmed in sight by the First Officer. At that moment the aircraft was levelling of at 6000ft (speed 190kts) and rolling out of a 15deg left turn (roll rate 2deg/sec) with flaps at 1, the gear still retracted and autopilot and auto-throttle systems engaged. The aircraft then suddenly entered the wake vortex of a Delta Airlines Boeing 727 that preceded it by approx. 69 seconds (4,2mls). Over the next 3 seconds the aircraft rolled left to approx. 18deg of bank. The autopilot attempted to initiate a roll back to the right as the aircraft went in and out of a wake vortex core, resulting in two loud "thumps". The First Officer then manually overrode the autopilot without disengaging it by putting in a large right-wheel command at a rate of 150deg/sec. The airplane started rolling back to the right at an acceleration that peaked 36deg/sec, but the aircraft never reached a wings level attitude. At 19.03:01 the aircraft's heading slewed suddenly and dramatically to the left (full left rudder deflection). Within a second of the yaw onset the roll attitude suddenly began to increase to the left, reaching 30deg. The aircraft pitched down, continuing to roll through 55deg left bank. At 19.03:07 the pitch attitude approached -20deg, the left bank increased to 70deg and the descent rate reached 3600f/min. At this point, the aircraft stalled. Left roll and yaw continued, and the aircraft rolled through inverted flight as the nose reached 90deg down, approx. 3600ft above the ground. The 737 continued to roll, but the nose began to rise. At 2000ft above the ground the aircraft's attitude passed 40deg nose low and 15deg left bank. The left roll hesitated briefly, but continued and the nose again dropped. The plane descended fast and impacted the ground nose first at 261kts in an 80deg nose down, 60deg left bank attitude and with significant sideslip. All 132 on board were killed.

More information

From 737 Systems Website:

The main rudder PCU contains a Force Fight Monitor (FFM) that detects opposing pressure (force fight) between A and B actuators. This may occur if either system A or B input is jammed or disconnected. The FFM output is used to automatically turn on the Standby Hydraulic pump, open the standby rudder shutoff valve to pressurize the standby rudder PCU, and illuminate the STBY RUD ON, Master Caution, and Flight Control (FLT CONT) lights.
The standby rudder PCU is powered by the standby hydraulic system. The standby hydraulic system is provided as a backup if system A and/or B pressure is lost. With the standby PCU powered the pilot retains adequate rudder control capability. It can be operated manually through the FLT CONTROL switches or automatically. (Refer to Chapter 13, Hydraulics, Standby Hydraulic System)
An amber STBY RUD ON light illuminates when the standby rudder hydraulic system is pressurized. The standby rudder system can be pressurized with either Flight Control switch, automatically during takeoff or landing (Refer to Chapter 13, Hydraulics, Standby Hydraulic System) or automatically by the Force Fight Monitor. The STBY RUD ON light illumination activates Master Caution and Flight Control warning lights on the Systems Annunciation Panel.