I received a brand-new 2016 rMB earlier today, and as I was setting it up, I noticed how I kept on getting my password wrong for my Apple ID. Somehow I managed to get it right after a couple tries, and I opened up Notes to see whether there was a keyboard issue. I noticed that I have to press the left shift key in a certain manner - more specifically, I need to press the key as close to the center as possible. Pressing it down at the left or right did not activate it when I tried to type an uppercase letter. However, the right shift key seemingly works as intended; I can press the left or right side of the key, or the center, and I get all the capital letters I want. Did I receive a defective unit?