The 21.1MP Canon EOS 1Ds Mark III apparently has the same auto-exposure bug I see sporadically on its 10.1MP sibling, the EOS 1D Mark III. I don’t have an explanation as of yet, but in some cases (with the 1DM3) I’ve even been able to reproduce the problem—it is not user error. In this case, I had even dialed in -1/3 stop exposure compensation.
Gross auto-exposure error—Canon EOS 1Ds Mark III
Proper exposure—Canon EOS 1Ds Mark III