Upload & Sell: Off
It can say same, unfortunately err99 can say numerous other things. It simply says "that photo isnt possible right now".
That is no longer the case with the latest firmware (I have 1.2.5 in my 1D Mk III). Error 99 was split up into half a dozen codes starting with 80, and rather foolishly reusing 99 as a specific error. I don't have a bookmark handy on this machine, but I think they are listed in the firmware release notes. Error 99 now means the shutter can't complete its cycle.
If you don't have the latest firmware I'd put it on there. And I'd do it with the AC adaptor rather than the battery if you have that (it came with my camera new).
I've had an Error 99 in one session and it was caused by a bad battery. That battery (genuine Canon and less than a year old) was known to have high self-discharge on another body and barely gave 500 shots on the other body. On this occasion, I was tracking a bird in AI servo at high frame rate with a super-tele, converter, IS running, early in the morning in 4°C. A quick search showed that the shutter sometimes can't complete its cycle if the power sags in the middle.
Clearing the fault with a good battery did require removing the CF card and cycling the shutter without it ("shoot without CF card on"). This was part of the Canon diagnostic procedure I found on their site.