On my case, it's its a stucked-up aperture. i have a Canon 350D rarely used. Out of storage one day error 99 showed up. After trial and error (changed CF card, cleaned contacts for Clock battery), i changed lens and found that my stock lens was causing the error. I then proceeded to manual settings and set my aperture to bigger value, took a picture and VOILA! the error was gone. It seems the long storage made the aperture contacts a bit stiff :-)