Mission controllers at NASA are currently working to return the Curiosity Mars rover to full activity after a computer glitch that caused the rover to enter into a precautionary stand-down over the weekend. Image: NASA/JPL-Caltech/MSSS
It doesn't appear to be serious, but NASA engineers are still trying to find the cause of Curiosity's safe-mode entry. The rover ceased its regular activities on July 2, reverting to a minimalist operating mode. It's currently going through a prescribed sequence for resuming normal functions. Curiosity is currently stable and communicating with its NASA controllers.
NASA isn't sure what caused the software glitch, but preliminary data suggests an "unexpected mismatch between camera software and data-processing software in the main computer". So it sounds like Curiosity has an acute case of previously undetected computer bug. Note to NASA: You might not want to perform that particular function again -- at least not before performing a software upgrade on the vehicle. (Yes, a long-distance firmware upgrade is indeed possible, and it has been done before.)
As it reboots and works to resume full activities, the rover will be able to transmit more diagnostic information to its controllers. We'll probably know more in a few days.
This isn't the first time Curiosity has had a software freakout. The Martian probe has entered into safe mode on three previous occasions, all of them in 2013.