nikon: What could make the mirror of a D7000 flutter and show "Err" when trying to take a picture?

This is very likely a power problem. If the battery supplies an inadequate voltage to the camera, the camera may not perform all the necessary tasks to take a picture, and opening the shutter is one of the most energy-intensive tasks.

If the battery was not fully charged, charge it and try again. If that does not work, buy a new battery, charge it fully and try again. If you're lucky, a nearby camera store may have a battery that can be used for the test. (If so, buy your battery there, if your test is successful, thank the store for allowing you to experiment).