My D750 is about a year old. Last summer I noticed the camera would lock up and show an "err" message. The mirror was locked in the up position. The usual solutions ( battery removal etc etc) did not work. I did not think much of it at first but the problem started almost on a regular basis. It appears when the camera has not been used for awhile it will happen. Pushing the shutter release button all the way down "fixes" the problem.
Nikon "repaired" it by replacing the front cover, some rubber grips and new firmware. It still acts up and seems to happen more frequently ( not just first shot of the day)
To say the least I am utterly disappointed with the camera. The D750 has had several recalls and I think for a $2500 camera you would expect better.
Yes the images it produces are great but I have lost a bit of confidence in the longevity of the camera.
A Google search for D750 err messages will show this is not an isolated issue.
It will go back to Nikon Canada and I hope they will fix it this time.