Well that's interesting. I haven't seen this reported on the F8500 (it mostly seems to be a F5300 thing) but my July 2014 build 51 inch has a faint pink band along the bottom of the screen. Maybe about an inch or two from the bottom and about three inches tall. It's visible on gray, white, and green color slides and I just saw it in some normal content that had a full screen image of light blue sky. It almost looks like the wide white banner that CNN uses along the bottom but it can't be IR....I've watched about an hour of CNN on the TV at most.
As defects go it's very minor. I'd say the color is similar to the slight pinkish hue I get in the upper right on full screen white. But being a fairly well defined band with visible edges rather than a 'splotchy' area the eye is drawn a bit more to it. In any case the location is a good one....it would be well 'out of the way' when watching letter boxed movies.
I certainly don't think this developed over time (I'm probably just now noticing it and it's possible that my recent calibration brings it out a bit more). My panel has 500 hours on it.
This is my first plasma so while I'm a bit dismayed to find that their 'perfect uniformity' is anything but this TV wows me on a regular basis so I think I'll keep it.
My ST60 had a pink blob in the top/middle that was much worse.