Alright, really quick. I was using my Nikon D7000 for a great shoot and everything was going great until at one point the battery ran out while I was shooting video in Live View and then boom "Err" message.
This came up because my mirror was stuck. Lots of people say you need to send it back to Nikon, but I plugged in a new battery, and shot a picture (to reset the mirror) and all was good.
So if you get this message and your mirror is stuck, just try putting in a new battery and click the shutter button to reset the mirror. It just might work.
Alright that's it for now. Thanks for reading. Hope this helps out some peeps.