I've was bitten by this in the first month we had our 5D3. I put a CF card in and the camera wouldn't function. It would turn on, but then nothing could be set (e.g., ISO, aperature). Movie function wouldn't turn on. When I turned it off, it wouldn't come back on. Pulled battery and same thing. Called Canon. They were no help, but did issue an Repair ticket immediately. Just after hanging up with them, I pulled the CF card and battery. When I put only an SD card in, the camera operated fine (with battery, of course). When I looked into the CF slot, I could see the bent pin touching the side of the slot and, apparently causing a short.
Canon repaired it quickly under warranty. Total turn around, including shipping, was about a week.
Bottom line: I put CF cards in very gently now.
It seems that if the problem is so pervasive that Canon might consider a recall.