What causes a fluorescent bulb to fail?

When an incandescent light bulb fails, my understanding is that the filament has finally, and literally, burned out and ceases to complete the circuit. What fails in a fluorescent bulb when they “burn out”?

http://en.wikipedia.org/wiki/Fluorescent_lamp#Mechanisms_of_lamp_failure_at_end_of_life

I’m not sure which failure mode is most common.

N/M…scooped.

Cool, thanks!