Monday, I was out shooting with the 1DX and MKIV and 500mmII. I was comparing the MKIV to the 1DX with the 1.4XIII attached. Everything was on a tripod and Wimberley. I set "both cameras to RAW only."
I shot about 500 shots with the 1DX for about 3 hours and when I got home the 1DX had "50 RAW files at 20MB each" and "450 Jpeg S files at 1MB each." I was in shock!
I figured I must have accidentally changed the setting from RAW to JPEG. However, If you look under the 2nd red menu buttons under (SHOOT2) this seems somewhat impossible, as you have to press a few buttons for this to change, so I thought.
So, I discovered that if you press the "Card Image Size Selection Button" (below the Image Playback Button, Pg. 117) and then turn the Main Dial only one notch to the left, you will change your image choice from the highest quality RAW to the lowest quality Jpeg!
I must have accidentally changed to Jpeg when I was taking the 1DX off of the 500mm lens/extender combo in order to mount the MKIV. Personally, I don't think that Canon should have the original setting as the default, or at least require you to press the SET button.
Everyone should try it now, so that you'll know what I'm talking about! Since I was comparing the MKIV and the 1DX, I was so focused (no pun intended) on framing the shot and setting the same exposures via the histograms that I didn't notice the change. After-all, I set both cameras to RAW and confirmed the word RAW on the Histogram screen! I can only imagine a wedding photographer accidentally changing this setting in the dark and not realizing it until the next day.
The good news is that you can change the "default" image size selection method to a different custom selection method.
Go into the orange menu; C.Fn4: Disp/Operation; it's the last choice, "Rec card, img size setting. You could change this default setting from "Rear LCD Panel" to "LCD monitor." Now, if you accidentally press the "Card Image Size Selection Button" you will see the setting change on the large LCD screen as it lights up, as opposed to the small dark LCD screen. For those that haven't noticed yet, the white menu choices are the defaults and once you choose a custom selection, then your menu choice turns blue.
I never accidentally changed this setting on the MKIV and it's "function button" appears recessed and on the 1DX, it's not. But for those interested, to change from RAW to Jpeg on the MKIV it's the function button next to the trash can and then one notch turn of the main dial. To prevent this from happening on the MKIV, you can change the default in C.FnIV:8-1 from rear LCD to LCD Monitor.
I hope that this info will save somebody one day! Luckily my test shots were not important, and they still looked pretty good considering they were only 1MB.