From Canon website:
Why do I experience inconsistent Auto Focus (AF) behaviour with my EOS-1D Mark III?
Phenomenon:
The focus or AF tracking is inconsistent when using the Artificial Intelligent (AI) servo continuous shooting mode under high temperatures in particular.
Cause:
In the digital SLR camera, EOS-1D Mark III that was launched in May this year, Canon has found that some cameras have an adjustment problem with the mirror for AF. The adjustment problem means for the affected products that the focus is not stable or that the Auto Focus tracking is not right when using the AI servo continuous shooting under high temperatures in particular.
Affected products:
The products that could have this problem can be identified as explained below. If the serial number on the bottom of the camera is between 501001 and 546561, it could contain one of the AF mirrors with the adjustment problem.
Support:
See Canon Website
