It is unfortunately common to blame the unknown/unexplainable on a bug, which sometimes includes accusations in regards to a new firmware update. If, as was suggested, you do some youtube searching and internet sleuthing, my bet is that you'll discover no one has yet looked at the .DAT or .TXT files (i.e., the flight log) and discovered an ESC error, a failed motor, a battery that was used "down to the nub," etc. (Not to mention that most famous of bugs - pilot error!
Classic (and mentioned already in this thread) - fly a quad out of sight. Go into semi-panic mode (or full-blown panic, choose one!). Press RTH and start praying. Panic continues. Operator fiddles with buttons, joysticks, views, etc., and accidentally turns off RTH. As another poster also wrote - now the quad hovers (possibly *still* out of sight) until the battery reaches critical voltage (controllable, for the most part, by the operator/owner) and lands. Or at least, tries to land. A quad landing somewhere where you cannot see it is called a crash. Yes, TRACKIMO (or similar), the pilot log and/or cached video on your tablet/phone/device will often help you find where it landed (if it didn't really crash).
When things truly go haywire - quad lands 200 yards offshore at the beach - then you truly are going to experience a bout of bad luck. The quad "world" is full of people who unbox the quad, install appropriate software, log on, make half a dozen flights, take on the attitude of "...hey, this is easy, I got this..." and then fly up to 1,000 feet (into a much stronger wind they never realized was there), fly downwind, blah, blah, blah. The quad is now most likely already out of sight. They know how to fly in ATTI mode (we think) but, by this time, the quad is virtually invisible and extremely difficult to identify front or back. Now something minor goes wrong, but you're already in no man's land and the whole scenario ends in at least a minor disaster.
Most of the readers here know exactly what I'm talking about and they, too, could write a book about why things go wrong. Please, please, don't ask *me* how I know some of these things! LOL
Art - N4PJ
Leesburg, FL