Originally posted by StreetShooter
The older versions (> 1-2 years) of this lens quite consistently give an "Error 99" with the 10D when used at any aperture other than f2.8. This was a problem with the EOS 30 as well, from what I understand. See dpreview.
The newer lenses would have addressed the problem with an upgraded chip, before being sold. In other words, yours is probablu a relatively new lens. Sigma's policy is to upgrade your lens chip for free if you find any compatibility problems with newer camera models.