One interesting thing I notice from time to time are these dark spots forming in CFLs, they seem to occur more frequently in the PL type lamps, but ive only ever noticed this in compact fluorescents, can't say ive ever seen this happen in a linear, non-self ballasted lamp. The pic below is one I noticed today, this one isn't that bad. The worst ive seen was a GE Biax PL with nearly 1/4 of the tube (near the U-bend) completely blackened and producing a faint blue color out of only the black area. Anybody know what causes this, and why it only seems to affect self ballasted CFLs?

I have seen this appear I'n the 26w tri tube CFLs in the recessed cans from time to time, normally from constant use. I did have two westies that were severely blackened at the ends and also showed the blue glow through some breaks in the phosphor.

Blackening at the ends is "normal" sputtering from the electrodes (mercury, emitter remains, tungsten, support wire material).
But being in the middle of the tube and so much concentrated on such small spot looks like some contamination defect destroying the phosphor there.

It can be even a loose amalgam pellet, where some of the components reacts with the phosphor...

this happens a lot on spiral clf (almost every spiral cfl i have/had this issue).