If like me, you usually carry 2-3 camera bodies on any given assignment, then there's a chance that the one you have around your neck, may experience what mine did!
After shooting the Vanier Cup Football game, I transferred the CF cards from each of the three (3) camera bodies, only to noticed that on one (1) of the cards, the files changed from Full RAW to Small JPEG, midway during the game. How was that possible!?!
Was it the cold weather, had someone in the press room tampered with my gear? It was a complete mystery, until I reconstructed the pieces of the puzzle. It was the camera body around my neck, with the smaller lens mounted on it (in this case a 16-35mm).
If the event you are covering has you running around, and you are carrying a camera around your neck, (along with others), and that you have a chest vest, the camera, while bouncing around, can accidentally hit the "File Size" button, and toggle from one mode to another. Mystery solved!
I haven't thought of a quick fix to prevent this as of yet, but will inquire with Canon, to see if there's a way to dis-engage that button, through firmware. I'll keep you posted, meanwhile, I'd suggest you keep an eye out, and verify that the file size you've selected to shoot with, hasn't changed... "On it's Own".
Update (Nov. 25th 2013) - After verification with Canon's Technical Support team, as of yet, there's no way to disable this button. However, they have passed the information along for future consideration.