Edit: as usual, a typo :-/ it’s the EOS-1D X we are talking about here
Canon released a firmware update for the Canon EOS-1D X, version 2.0.8 (download).
Firmware Version 2.0.8 incorporate the following fix. Fixes a phenomenon which occurs when the "Shutter button half-press" menu item, configured in "C.Fn5: Operation" > "Custom Controls Menu", is set to metering start only; depending on when the shutter button is half-pressed, it may initiate AutoFocus. Firmware Version 2.0.8 is for cameras with firmware Version 2.0.7 or earlier. If your camera's firmware is already Version 2.0.8, it is not necessary to update the firmware. The firmware update takes approximately seven minutes.
Notes: 1) Once the EOS-1D X camera is updated to Version 2.0.8, it cannot be restored to a previous firmware version (Version 1.0.2 through 2.0.7). 2) It is recommended that the latest application software*1 be used, once the EOS-1D X is updated to the firmware Version 2.0.3 (or later) from older firmware (Versions 1.0.2 through 1.2.4). Certain previous versions of these applications do not support functions which are added by the new firmware. You can download the latest application software from our Web site. *1: EOS Utility version 2.13.30 or later *1: Digital Photo Professional version 3.13.50 or later 3) Accompanying the enhancements to the functions, the instruction manual has also been revised. Users are asked to download both the latest firmware and the instruction manual. You can download the latest instruction manual from our Web site. 4) As a result of this fix, cameras with Firmware Version 2.0.7 (or later) will not be able to read the "camera settings file" created with cameras running Firmware Version 2.0.3 (or earlier). If you require the "camera settings file", please create it after updating to firmware version 2.0.7 (or later).