As a former choir student, I can attest to the fact that when performing in front of people, the lyrics can tend to leave your mind. Luckily for choir students, if one person forgets the lyrics, there are dozens others to continue on singing. When you're singing solo, this is not the case. With that in mind, I don't fault Aguilera for forgetting the lyrics. She has performed the National Anthem before at public events and remembered all the lyrics, so it's not like she simply doesn't know the lyrics. She was probably just very nervous to sing for the Superbowl. I imagine that is a very nerve-racking venue.
Now, for the actual issue at hand here. Over-singing. I like powerful singing every now and again, and I don't really mind powerful singing when one is singing the national anthem, but over-singing is a problem. To me, when singers belt out the national anthem, making a monosyllabic word have 15 syllables ( "...star-spangled banner yet wa-a-a-a-a-a-a-a-a-a-a-a-a-a-a-ve..." I might have missed a few), is disrespectful. When a person becomes so obsessed with showcasing their voice it gives the impression they care more about their voice than they do about the country. Sing it powerfully (Faith Hill), fine. Over-sing it (Christina Aguilera), no thanks.
Of course, there are people who will say "This is America. Freedom of expression. People can sing it however they want!" Okay, first of all "freedom of expression" is not in the constitution. It's freedom of speech. Second of all, what comes to my mind when people say that is the song Taps. Yes, there is a lyrical version to that song: Lyrical Taps Would you want Aguilera to stand at a soldier's funeral and belt that song out like she is singing at a concert? I sure wouldn't. It'd be disruptive and annoying. Likewise, I don't want her to belt out the National Anthem.
However, since the National Anthem tune was stolen from a British drinking song, I don't mind people being lively in their singing. It is a triumphant song after all (Woohoo! Waving flag after a battle in the war of 1812! Live on American Spirit!). But triumphant or not, a national anthem deserves a certain level of prestige and respect. The song should be about our country, our citizens, our American spirit. It should not be about some celebrity's ability to sing so loud the starving children in Africa can hear her.
And on that note, a lovely rendition of the National Anthem: