http://www.jpl.nasa.gov/news/news.cfm?release=2009-182  

Another Stall of Right-Rear Wheel Ends Drive
Jet Propulsion Laboratory
December 01, 2009

Spirit's right-rear wheel stalled again on Sol 2099 (Nov. 28, 2009)
during the first step of a two-step extrication maneuver. This stall is
different in some characteristics from the stall on Sol 2092 (Nov. 21).
The Sol 2099 stall occurred more quickly and the inferred rotor
resistance was elevated at the end of the stall. Investigation of past
stall events along with these characteristics suggest that this stall
might not be result of the terrain, but might be internal to the
right-rear wheel actuator. Rover project engineers are developing a
series of diagnostics to explore the actuator health and to isolate
potential terrain interactions. These diagnostics are not likely to be
ready before Wednesday. Plans for future driving will depend on the
results of the diagnostic tests.

Before the Sol 2099 drive ended, Spirit completed 1.4 meters of wheel
spin and the rover's center moved 0.5 millimeters (0.02 inch) forward,
0.25 millimeters (0.01 inch) to the left and 0.5 millimeters (0.02 inch)
downward. Since Spirit began extrication on Sol 2088, the rover has
performed 9.5 meters (31 feet) of wheel spin and the rover's center, in
total, has moved 16 millimeters (0.63 inch) forward, 10 millimeters
(0.39 inch) to the left and 5 millimeters (0.20 inch) downward.