Have you guys never heard that singing in a football stadium is the worst thing a singer can do? The acoustics are such that when the singer starts singing the second line, the FIRST line starts bouncing off the far wall of the stadium, a mile away and echos back at the singer.
Their impulse is to stop singing till the sound "catchs up" with them, but of course they can't do that because then there'd be dead silence. That's probably why Christina sang the same line twice. She got caught up with hearing herself. She was supposed to keep plowing through, and
just keep singing. Either no one told her what would happen, or she didn't bother to rehearse enough to know what to expect and sing through it all.
One of the few and best renditions of the national anthem was when Whitney Houston did it and did a fabulous job. Of course, then she got slammed,

as she had pre-recorded herself to listen to while singing, put in an earphone in one ear to sing along to herself and put an ear plug in the other ear so she wouldn't hear the echoing.
People just can't win with the national anthem. It's all of ours, as everyone has an opinion about it. Some want to hear something other than the same old, same old. Others believe: don't screw with our national anthem.