Here's a typical example of such an erroneous black frame (photo taken in bright sunlight!):
This is from the official Nokia Discussions forum (via Flickr):
Why does my device capture dark images after manually settings the ISO setting in Creative mode? - Nokia FAQ
Nokia 808 PureView is built to take great pictures easily in Automatic mode. If you want to get creative in bright light conditions, our recommendation is to set the ISO setting into the default mode i.e. to use Auto ISO in Creative mode. Within upcoming weeks we will provide software update that will allow to use ISO setting in bright light conditions more flexibly.
Which is a little disingenuous, seeing as the whole point of Creative mode was that you didn't have to leave everything on 'Auto'. Still, at least the bug is now acknowledged, it sounds like it's in firmware rather than in the image processor itself, and the timescale quoted is 'weeks', not 'months' or 'never'....!
Here's the proof: