"Power button HID event - flushing proactively" -- what does this mean?
A little Google searching led me to the fora at MacRumors with this tantalizing comment and log file excerpt:


  Ok, what’s in this log is not the kernel panic on boot but apparently
  the last thing the computer was doing when my mother started having
  problems (Safari seemed to freeze up) and she killed everything by
  long pressing the power button.
  
  Nov 15 15:06:52 macintosh logd[75]: Power button HID event - flushing proactively