I shoot my D750 mostly sports with my 300mm f/2.8. Because sports pictures are hit and miss with focus. I accepted the fact the a portion of the photos are just out of focus, period. But it seemed like I was having more and more OOF shots. Until few months ago, i used on some stationary subjects, then i noticed a consistent back focusing on all the shots with stationary subjects. Ran some studio tests, then confirmed it is back focus even with different lenses. Tried the d750 calibration method with no luck. I think I went all the way -24 setting and it still back focuses. Nikon says if the calibration cannot resolve the issue, then there is a problem with the camera focusing function. Looked up the Nikon service site, they want $450 for such repair. Then I came across Nikon D750 shutter recall on the dark bands issues. So I sent it in for the shutter repair and wait and see what happen. I got the camera back in about 3 weeks (damn Nikon's poor repair service turnaround time, forced me to buy a used d800 while the d750 is in the shop). surprisingly the back focus is fixed as well as they put in a new shutter. Now it gives exceptional sharp images with the 300mm f/2.8. I doubt that they looked at the focus mechanism. My guess is the contacts of the camera somehow dirty or not communicating with the lenses. For them replacing the shutter/servicing the body, they probably cleaned the contacts (or whatever that communicates between the body and the lens. I'm happy that my pictures are sharp again, and got a new shutter too.    So what do you think that is? Has anyone have a similar problem?

the only down side is now i have an extra d800. what a memory card hog that is. With a 8 gigs CF card, it's like shooting a roll of 36 exposure film. Before you know it, your card is full.

If they replaced the shutter, they have to pull out the mirror box along with the AF mirror.  Putting it back together will call for the focus and parallelism of the bayonet to the sensor to be checked too which affects the focus.

They likely tweaked the AF mirrors cam a bit on reassembly to correct the back-focus.  I did it on my D800E a couple of years ago as it was also up against a -20 at times with back-focusing issues on the 70-200mm lens and a little less with the others.

Turn around time seems normal.  I often haul my stuff the the LA shop so it saves me a few days of shipping and usually comes back within a couple of weeks.  Much longer if the NY office gets involved with an estimate via snail mail and approval thing between NY and LA service.

Good luck!