NaN's (not a number)

I first came across this term I stared blankly into the TD's face feeling lost ..."what's a nairn ? " - thinking of Garlic Nairn bread...

Recently they copped up again whilst I was testing my Bullet coin sim  - sometimes I would re-render a small window around them in the render view / IPR environment and they would dissappear - other times they would stick around.

So i dug up some info: (*to be completed soon.)