As crazy as it may sound Corsair XMS series RAM, seems like every build Core 2 to Core i series I seem to hit POST issues with stores who get mixed "batches". Oddly I never have problems with their "value/ghetto RAM", for high performance memory I mainly stick with GSkill & Kingston models with heatsinks.
2nd worst failure is Intel HSF, I've gotten a few duds which delayed builds. Most projects I tell clients they'll be footing the bill for Arctic Cooling HSF as the stock Intel ones are unreliable junk.
3rd worst, I actually had a PCI 4-port USB card meltdown/catch fire during a burn-in test... *cues X-Files theme*