In article ,
(Samuel M. Goldwasser) writes:
See: http://repairfaq.cis.upenn.edu/Misc/cflhole1.jpg

This is a ~1 mm hole in the glass near one of the filaments.
Something got hot enough for the glass to melt, and after
that, as they say, the rest was history.

I've seen this on 3 CFLs in 3 different lamps/fixtures. There are no known
problems that could account for such nasty behavior. They were all high
mileage, so perhaps the filament at that end of the lamp opened resulting
in the discharge going to one post, near the glass, or something.

The CFLs were all from GE but I don't know if they are of the same
ballast/lamp design.

Comments welcome.

Caused by control gear which fails to detect when the tube has reached
end of life (emission material all sputtered off), and and provides enough
voltage headroom to continue driving the tube as a cold cathode tube,
which generates too much heat at the tube electrodes. Dead thermionic
tubes don't last long when driven as cold cathode tubes, because the
filaments and support wires are quickly burned away, until the tube cracks
and vents to the atmosphere. If the support wires are very close to the
glass tube wall, as is likely with thin tubes, it can melt the glass.
With linear tubes, it can cause the tube to break such that it drops out
of the fitting - I've had one case of this. This was in a set of T4 tubes
which come in various lengths from 6W to 24W, and all use the same control
gear. I had mostly 16W ones, and when the tube reaches end of life, the
control gear simply up's the tube voltage to maintain the current, and the
excess power is dumped into the (now) cold cathode electrodes, which start
glowing as bright red hot dots, clearly visible through the dark sputtered
coating which is now round the tube ends. The heat also did enough damage
to the plastic lampholder that it was no longer usable. I did think this
could be a fire risk too, particularly if there had been any flammable
material nearby. (I've since been phasing these lamps out of use, which
is now forced as the manufacturer has gone and no spare tubes available.)

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]