According to Canon Rumors, a firmware update is available from Sigma for the SIGMA 120-300mm f/2.8 DG OS HSM for Canon.
Firmware does the following
It has improved the Auto Focus speed of “Standard” and “Speed-priority” modes.
It has improved the focus accuracy when it is mounted on Canon EOS-1D Mark III or Canon EOS-1D Mark IV
Link
http://www.canonrumors.com/2014/06/sigma-120-300-f2-8-dg-os-hsm-for-canon-firmware-update/
Firmware does the following
It has improved the Auto Focus speed of “Standard” and “Speed-priority” modes.It has improved the focus accuracy when it is mounted on Canon EOS-1D Mark III or Canon EOS-1D Mark IVLink
http://www.canonrumors.com/2014/06/sigma-120-300-f2-8-dg-os-hsm-for-canon-firmware-update/