I have experienced (once) a seize up with the EOS-1D X Mark II and Mark III bodies when firing in the maximum burst mode, in severely cold weather. A quick pull out and reinsertion of the battery cleared the problem immediately.
In addition, there is a phenomenon, which I have not experienced, as described below :
When the camera is used with the electronic level set to “Show” in the viewfinder display, in rare cases, the camera may not perform normally when operating the AF-ON button or shutter button.
EOS-1D X Mark III (Firmware version 1.0.0)
Use the “Hide” setting for the electronic level in the viewfinder display.
Setting procedure: Menu / Set-up 2 tab / Viewfinder display / Electronic level* [Hide]
*Default setting: [Hide]
In early-April 2020, Canon plans to release a firmware update to address the phenomenon listed above.