While the rovers are doing very well, it seems NASA may have still managed to mess them up:

NASA's Mars rovers Opportunity and Spirit are identical twins - so alike that they even fooled NASA. Researchers have discovered that they sent the robots to Mars with an instrument meant for Opportunity inside Spirit and vice versa.

Turns out it's actually the same instrument on both rovers, it's just that each had a slightly different calibration. By using the wrong file, small errors started creeping into the data. However, now that it's been spotted, a fix is apparently very easy.