Module 4, one of Kepler's 21 CCD Modules, retired on July 20, 2016. K2fov v5.1 was released to incorporate the loss of this module. Targets falling on this module, i.e. targets falling on channels 9, 10, 11, or 12, are no longer considered to be "on silicon" as of this version.
More details: http://keplerscience.arc.nasa.gov/retirement-of-ccd-module-4-confirmed.html