I've heard of this but it hasn't happened to me. However, I have had a "sleep of death" issue that appears to have come up for a lot of people since updating to the 6.2 software. If I leave the KF sleeping and unattended for a long time, it sometimes doesn't wake up when I press the power button. I have to hold down for ~10 seconds (which forces it to power off, like pulling the battery), then press the power button for it to start up again.
Hopefully this gets fixed. Apparently 6.2 fixed some of the touchscreen responsiveness issues but I guess it added this extra problem. Good thing is that Amazon has shown they're committed to supporting the device with ongoing patches.