Don't fix it:

The European Space Agency’s (ESA) SMART-1 Moon probe had a bit of unexpected engine action. Spacecraft operators last month were surprised to find the craft’s ion motor happily at work.
...
The event seems to be related to a shift of memory addresses caused by the uploading of a major software patch. The patch was not uploaded to fix a problem, but to provide new functionality for the lunar orbiter’s science instruments, Grahn told SPACE.com.

At least it didn't crash into the moon because someone forgot to convert standard to metric.