This panel-mount LED indicator glued to the Z-axis stage of my Thing-O-Matic had been dutifully showing a bright green glow when the extruder heater was active:
Of late, it began flickering erratically whenever the heater turned on. It used to flicker when the PID loop (hacked to be a bang-bang controller) drove the extruder temperature past the switching threshold, but this was worse.
It’s rated for 5 VDC, 25 mA and has an internal resistor to make that happen. Channeling the true spirit of DIY 3D printer electronics, I deliberately connected it directly across the 12 V extruder power and let it burn at 80 mA. The poor thing was surprisingly bright for an ancient green LED ( the 8124 date code stamped on the side I pried off for the picture says it’s three decades old) and, even under that abuse, it lasted for a year: not to be sniffed at.
I’d expect the LED to fail open when a bond wire burned through, but you just never can tell. It worked fine on the bench, which is typical of all intermittent failures.
So I popped an identical indicator off the stack, conservatively added a 270 Ω series resistor to drop the excess voltage, and it’s all good again.
Ya gotta have stuff, right?
The Smell of Molten Projects in the Morning 
Yes I have seen LEDs flash at a few Hz when they fail. I assume the bond wire must make and break due to thermal expansion.
It really spoofed me for a while: I thought the extruder controller was going toes-up in a spectacular manner.
Fortunately, the last electronics bay rearrangement put the EC directly under that LED, so I could watch both LEDs at once (which would have been easier with independently aim-able eyeballs, like a lizard). Turned out the EC LED behaved properly while the panel-mount LED flickered wildly…
Elektor had a brief mention of LEDs flashing when they fail, entitled “A Ghost in the Machine”. Abstract here: http://www.elektor.com/magazines/2011/september/a-ghost-in-the-machine.1912289.lynkx
A few years ago I bought a batch of white LEDs back when they were still something special. Many of them failed, and all showed their upcoming demise by flickering. Very annoying if you’ve built a lamp with them of multiple series-strings and cast it in epoxy resin for waterproofness….
I have never had ordinary (red/green/yellow) LEDs fail by flickering, but I always drive them very conservatively (or miserly); I wouldn’t call 80mA as a driving it conservatively :-)
That matches what I’ve seen: old cheap white LED flashlights, the kind with all the LEDs in parallel and maybe a ballast resistor, develop a flickering LED after a few years.
Well, it was in the spirit of the thing… [grin]