When your Mac Refuses to Sleep.

The Symptoms:

You put your mac to seep, the screen(s) go black, but the machine doesn’t power down its drive(s) and sleep. Pressing a mouse button or keyboard key brings the screen(s) straight back up.

The problem persists through reboots, logouts, changing user accounts, deleting com.apple.PowerManagement.plist, and even doing a full SMC reset.

The Solution:

Go to the Printers & Scanners preference pane, and make sure you haven’t got a printer with a paused print queue, or a printer which isn’t actually connected any more, which you may have accidentally sent a job to, which the system is trying to find.