Spirit's broken wheel is a feature, not a bug
This New York Times article describes how Mars Rover Spirit's wheel, which hasn't worked since March 2006, ended up scraping Martian dust off a patch of silica -- and when silica is found naturally on Earth, it is in one of two environments that "teem with microbial life." Scientists are still investigating the silica patch it found to see what we can learn from it.
Also of note, this January will mark the rovers' fourth birthday on Mars, well in excess of their original 90 day mission.
Photo from Wikimedia Commons