I actually read parts of this thread before I watched her performance, so I was quite surprised to see how small (in my opinion) the error was -- she didn't make up entirely new lyrics or stop singing, she just repeated a line. It's one of the easiest mistakes to make. I've done it dozens of times as an actor (repeating a line that carried a similar emotion) and a pianist (repeating a phrase of music) so I can imagine how easy it would be to do as a singer. Practice has nothing to do with it.
I'm really impressed with how professionally she handled it. I don't think she was rude, or unpatriotic, or anything else. She made a small error, she carried on, she apologized -- perfectly gracious behaviour, I think.