I bought a new iMac at work to use as a development / CI server. Every morning when I get back to the office, it's incommunicado. It seems like it just turned off (doesn't wake up from bluetooth keyboard/mouse events), but when I hit the power button it doesn't turn back on. Holding power for 10 seconds to hard shutdown, then hitting the button again works.
Checking the first obvious spot, Console.app for kernel panics, yields nothing. I tri-zapped PRAM on Thursday, but it was dead again today. Last step is an SMC zap, then I'm out of ideas.