In the post-Christmas sales I bought a brand new Canon 5D MkIV body and had put through only a few images - a couple of hundred max. It was all going swimmingly until I used it to photograph an awards ceremony at the university where my partner works and was receiving a prize at. Knowing it was available light only and would probably be quite dim I was using the 70-200 f2.8 MkII lens on the 5DIV as my primary kit. Simply put, the buffer seems to have died... 
When I took a photo I got the wait... message for about 5-10 seconds while the card was being written to - that is an eternity when the Deputy Chancellor and your other half are trying to hold a frozen smile and everyone is wondering "WHY doesn't he take the shot?!!!"
I had 2 each of Sandisc Extreme CF cards and SD cards (class10), all formatted on the camera body. I had two batteries, both new and fully charged. Having realized this was an issue early in the evening I tried switching cards, individually and as a pair, then I tried one card at a time, then I tried changing to just RAW then just JPG and nothing worked. I tried reducing the image size to no significant effect. To my knowledge there is no control to change the performance of the buffer (I was NOT using dual-pixel autofocus, I might add).
In the end I took the one shot with my 5DIV and used my back up camera, the Powershot G5X (chosen for its size and wider focal range) to get a second shot.
Very frustrating...
I have taken it to the shop and they have sent it to Canon to look at after asking me all the usual questions... 
@ebiggs1 wrote:"I have taken it to the shop and they have sent it to Canon ..."
No big mystery. It broke. It happens. That is why we have backup gear for such an emergency. When I was working full time I generally had four cameras with me. Now I only take two but they are both 1 series. and, not only bodies but backup lenses too.
I agree it's no big mystery. I guess I am a bit in shock - in almost 37 years of using Canon camera gear this is the first time I have had to send in hardware for service. I am glad I got A shot of my partner's award and I got some good shots of the Vice Chancellor and other recipients too, so the night was not lost. 
I guess one COULD say that in fact this is a testament to reliability of the gear. Certainly, from my years in computers, I know that electronic components often fail in the early parts of their life. I am glad it did so so early in its cycle, so at least it is a warranty issue.
" I am glad I got A shot of my partner's award ...."
Fantastic. 
It will mean a lot, maybe more than ever.
