I've been lurking for a while so, here's my contribution!
First, I found this by chance when my internet disconnected and I had to reconnect. I was in an AB and got this connected so I rebooted my internet and came back. To my surprise, when I was disconnected a person had come because I guess it counted me as gone. I wasn't able to click the scoreboard or press SPACE+SHIFT to show it. But, my raid frames were showing an additional person in the BG, one other person noticed it and laughed with me. Although we lost, I was still able to contribute to the BG. My paladin is only level 83, but I only got 3 honor for losing. Anyways, imagine 14 people disconnect and then there will be 28 people in the BG. This doesn't work in RBGs though.
Hope it helps!