Err 99 is Canon’s 42 :P
Newest experience with the camera. Just now I had tried to shoot a timed shot in the full-manual mode, the shutter fired after the de-facto 10 secs. But instead of the image being displayed, the small LCD panel flashed “Err 99″.
At 2:30 AM, such a message isn’t exactly welcome. Tried again, same error. Power cycled the camera, still no change. Detaching the EF-S 18-55mm kits lens, and keeping the body pointed down (so as to keep the dust from going inside as far as possible), pressed the shutter. No errors this time, a burnt, over-exposed frame filled the display.
A closer look at the gold contacts on the lens showed up a faint finger-oil stain. Dabbed it off carefully and re-attached the lens. Tried to shot a snap. It worked! Goody! The staining must have happened while trying my hand at “reverse lens photography” recently 
Apparently Err 99 seems like a generic code to signify any error condition. I guess 99 is Canon’s 42 