Hi all!
Have this gremlin with my D750 on two occasions now.
While shooting one day I had a "card error" on my screen. The camera locked up for a bit and would not take any photos.
After speaking with Nikon service Australia they said it could be the cards or camera. Trouble shoot cards first and go from there.
I shoot with 2x Sandisk and/or sandisk pro cards and write RAW to each slot at the same time
Anywho... I had these cards replaced and all worked well for a month or two.
Saturdays wedding I had this happen again, for the second time now.
There was also another photographer there on the day which had not experienced it yet with his D750 and ironically it happened to him twice that day within 10 mins.
Really weird!

Has this happened to anyone before?
Nikon service doesn't think so