Transient hardware problems
In this case, mouse madness. I sit at my desk in the early am for a round of blog surfing and collection of my mail from the last day and suddenly the scroll wheel on my optical mouse is ... not ... responding ... to ... my ... finger. (Speak those words in a faux robot voice if you please!)
Of course my first thought is "Crap. Do I need to buy a new mouse?" My second thought is to unplug the wee beastie and plug it in again. Nothing. No change. Then inspiration strikes like a frying pan to the noggin. It's an optical mouse and with the low-angle early morning sun, the mouse is sitting in a pool of intense sunlight that it is streaming in nearly sideways. The optical reader on the scroll wheel can't read the scrolling since it's being overwhelmed by the sunlight.
Move the mouse. Problem solved.