Whenever I flip on my dimmer switch, all four dimmable LED lamps flash on in unison, blink off, and then come on at the selected dim level. This happens even with the dimmer slider maxed out, so it's not a matter of trying to drive the lamps with too little power, which I know can cause flashing and flickering.
The flashing happens only on cold start, by which I mean, if I've just turned off the lamps and then flip them back on, they come on directly with no flash. Wait just a few seconds after flipping them off – a bit past the time they take to completely stop emitting light – and they will flash when you flip the switch back on again.
I don't know what could be causing this particular failure mode, so I don't know where to start debugging this problem.
For what it's worth, I first noticed this behavior when testing after wiring in the third lamp. When only two were wired in, no flashing was apparent.
Gory details:
I have a Lutron Diva CL 3-way dimmer. The "CL" designates that it handles CFL and LED dimming.
It's driving 4 Cooper Lighting HALO 4" LED retrofit kits, model RL406830WH.
The dimmer's package insert installation instructions state it can handle up to 150 W of CFL/LED load. Each of my LED lamps draws only 13.5 W, so all 4 together draw less than 55 W. This is well within the operating range of the dimmer.
The exact LED lamps I'm using are not listed among the "Approved Bulb List for Wall-Mount CL Dimmers", but several similar models by Cooper Lighting are.
ETA: I still would like to know why I saw this behavior, but the problem appears to have been the dimmer.
Disconnecting 3 of the 4 lamps showed flash-on-start still.
I swapped the dimmer for a standard S_3 and had no problem.
I then swapped in another instance of the same model. (Though the sticker on the side gave a different hardware rev, and the assembly location was St. Kitt's rather than China.) The new dimmer caused no flash on power-on, but it couldn't dim the LEDs nearly as low as the first instance. Frustrating.
Best Answer
As I don't know the interns of this dimmer it might switch when the alternating voltage (and therefore the current with resistive loads) crosses zero. To detect the phase (crossing 0V will occur at 0° 180° and 360° which is again 0° - you might want to look it up here: http://en.wikipedia.org/wiki/Phase_(waves) ) and the type of load (e.g. inductive loads delay the flow of current depending on their inductance and the frequency in relation to the voltage. I could not find a text on wikipedia describing that so here's a plain image: http://people.sinclair.edu/nickreeder/eet155/PageArt/phaseInductor.gif
To detect the type of load it has to observe the mains line for a short period of time without regulating it.
The issue does not occur on light loads because most dimmers can handle inductive, capacitive and resistive loads quite well on low power levels.
By letting the dimmer "cool down" you may erase its short term memory so it has to resynchronize every time you switch it on.
Again: As the internals of this dimmer (or better: the two) are not known I can't provide a better answer. A newer hardware revision usually improves the product. Also chinese manufacturers tend to produce lower quality products as seen here: http://hackaday.com/2012/08/15/buying-cheaper-electronics-and-not-saving-money/