I got my (first) Nexus 6 last week. I opened the box and thought "The back has come unclipped, I didn't even know that they had removable backs". Sadly, it was immediately apparent that they don't and the back had come unglued before the box was even opened. I tried to push the back back into place but it bulged unhealthily so I gave up and returned it. I didn't even turn it on. It was a very disappointing start to a new and expensive phone.
My new one arrives tomorrow. I really hope the same thing doesn't happen to that.