Ok so... I've been meaning to bring this up to Lighting Gallery for a long time, for real. I'm talkin YEARS.
When I was a kid, although I didn't get along with my dad very well, he did show a good amount of interest in fluorescent lighting.
He would buy magnetic Preheat CFL's, and also Programmed Start CFL's... one of which really caught my attention. He had this particular CFL
for several years, and took it with him during each move we made from state to state. It was one of those GE triple U-Tube BIAX CFL's, with
the "M" shaped bends. For some odd reason, one end was completely blackened... literally as blackened as it could possibly get... while the
other end of course, was completely fine. Absolutely no cathode filament sputter from that end either... almost as if only one electrode led
start up, like it were operating DC instead of AC.
That particular lamp went EOL due to internal failure within the base, but it had been EOL for years... flickering as if it were magnetic ballasted,
but with noticeable/concerning flicker during each start up. It had nothing to do with that heavily blackened end.
This isn't the first time I've encountered something like this with a GE BIAX lamp... I've seen this a couple times at Walmart when I was a kid.
The jewelry counter... I saw preheat GE BIAX PL-S lamps with fully blackened ends... still going with no flickering whatsoever. Not even the
traditional strobe flickering known to occur with Preheat lamps close to EOL. No hotspot movement or pink/purple color either.
So anyways... anybody else discover this type of situation with certain GE CFL's?

It seems to me this might've been an issue with some of the early CFL's from GE. I've only encountered this with GE though... no other brand.

I’ve had this on GE Biax, but I don’t know ifs it’s what you’re referring to?
Mine were caused by a ball of mercury rattling around one of the domes in the ‘M’ bend! The lamps were in my ceiling fan in the living room, and the vibration of the fan when running caused a ball of mercury to dance around the M bend, staining the phosphor grey on the bend?

Most fluorescent lamps ends their life with only one end blackened: https://www.lighting-gallery.net/gallery/displayimage.php?pos=-201779

Yeah, but mine was mercury staining?, although I have seen what you refer to, which I’m guessing is some from of contamination in the phosphor post possibly?

@Angryhorse, yep I know exactly what you're talking about regarding the mercury ball rattling stains. I've got that occurring as well,
but mainly in my Sylvania Dulux CFL's, although not to the extent that you described. But no... the mercury ball staining is not what I'm referring to.
End of life end blackening... with one end fully blackened as if it would reach EOL at any moment. Severe end blackening, yet absolutely no sign of
hotspot movement, no sputtering or purple flashes during start up, and no orange flashing. It flickered heavily during start up, but subsided once
the lamp finished warming up. From what I remember, it was programmed start circuitry, but only the filament on the non blackened side would glow before
the lamp fired up. This lamp also had a flicker pattern similar to a magnetic ballast, although it was electronic.
One thing I forgot to mention is that I've seen that CFL before it began flickering like a magnetic ballasted CFL. And back then it was also brighter...
but it still had that blackened end.
I've also encountered GE BIAX Preheat PL-S lamps with one blackened end, but still somehow operating normally. I don't see this phenomenon
anymore, nor have I ever found the exact model CFL my dad had when I was a kid. But all of this makes me wonder if it's possible that GE reused USED
CFL tubes somehow during processing, if this makes any sense.
This is something I've wondered about for yearsss!