This appears to be what happened. I duplicated it last night when trying to figure out how it happened. I must have pushed that button by accident while i was turning the back wheel for adjusting the aperture when I was shooting.
Calling Canon Monday. according to what I have read, there may be a way to disable this
If you bump it, then it changes the CF card and not the C-fast. Thankfully, the C-fast had all the raws
OMG what a potential nightmare. been using 1Dx and 1Dx2 bodies since they came out...first time I have heard about this or experienced this
Why anyone would shoot a wedding with a non dual card camera without both cards recording full raw at the same time is mind boggling to me. I just got very lucky. The Small Jpeg files were 1.7-2.3 MB......too small.
I'm happy to see that this turned out to be your eventual issue. Would be scary if the body was doing this on it's own. I shoot a lot of pro sports and need the speed so sadly I can't have RAW's saved to both cards because it slows down the shooting speed. I learned about this too when I was shooting but lucky for me I was out testing a new lens at the time it happened. Would have been a disaster if it happened during a paid gig.....