It's the rpi's version of a BSOD!
So some of you folks may have seen this: http://elinux.org/R-Pi_Troubleshooting# ... ash_screen
It flashes at the very beginning of boot - but sometimes if your kernel image is f'ed up it hangs there... Happened to me a couple times.
So -- important question to the rpi community -- what do we call it??
I'd vote Rainbow Square of Death. Other suggestions? Maybe more accurate might be "factory composite test pattern" of death. But FCTPOD is a little drawn out... Discuss!