About 6 or 7 months ago I inadvertently put a D850 battery in my D500 and when I turned it on I got a blinking "ERR" on the top of the top LCD and the camera was a brick. I thought, "oh man, what is going on here?". I put the D500 battery back in and got the same error. I am thinking I probably damaged a circuit board or something. I even did a total factory reset with no luck. I put the camera away and only picked it up again today. By then all the batteries (including the EN-EL18) in the battery grip had discharged to zero. I put both in the charger and when the EN-EL18 got to about 30% i put it back in the battery grip and crossed my fingers. I turned it on and got the ERR alert again and I figured I had to send it back to Nikon, this right after they discontinued it! After about 10 seconds the ERR alert went away and it started working again. I still don't know what went on but I am not complaining!