That's what I was afraid would happen. While it's possible it's the cards, you and I use different brands and different speeds. Moreover, between us we cover the two major brands I suspect others are using (SanDisk and Lexar).
I've been toying with returning my camera, but while I suspect we're the first to see/report this issue, it's possible that we won't be the last.
It could be a mismatch somewhere between camera settings (ala the "orientation specific AF" and AFMA issue reported earlier). I'm wondering if we have any "unique" settings in common:
* Before I started my shoot, I changed C.Fn3 so that "Low Speed" was 6 shots/sec. I then shot at that speed. After the lock-up, this value was reset to 3, but other values I had set at the same time retained their new values (e.g., changes to the custom menu).
* I had "orientation linked AF points" set -- I always have this set as the only lens I've used with the 1DX (a 70-200 L IS II) doesn't need AFMA
* I was constantly switching back and forth between portrait and layout shots
* I was also switching between "spot" metering (which I have linked to AF) and "evaluative"
* the camera had been on for longer than ever before (probably about 15 minutes or so)
Other than those, I didn't do anything different than normal. As for humidity/heat, I'm in the SF bay area, the day was dry and cool(ish).
I keep my low speed set to 6 as well, however, I don't think I have seen any difference with the camera set to low or high. I'll try it out though.
I also use orientation linked AF points, none of my lenses need AFMA so I keep this turned off.
I also only use spot meter and the evaluative metering when I shoot in AV. Most of the time I use manual exposure.
I had been using my camera for about 3-4 hours before it locked up (671 photos before lock up). I was inside an air conditioned venue. Actually the only time I see issues with my camera seem to be when I'm in an air conditioned room. It always seems to perform flawlessly when shooting out under the sun.
I have changed just about every custom setting on this camera so it would be hard to pinpoint anything.
I doubt custom settings have much to do with it except the AFMA bug might possibly be related but here's some of the major changes I have made: Multi function lock, Custom Controls, Lock/record, shutter release time lag, spot meter linked to Af point, restrict shooting mode and drive modes, Record photos to separate cards, selectable AF points (eliminated non-cross type), orientation linked AF point (separate), optimizer off, long exp noise reduction off, high iso NR off, highlight tone priority off.....