What shocks me most about Xtina's flub is that she has to have been singing that song for most of her life. Her childhood was jam packed with public singing appearances on the way up, including the whole "little girl prodigy sings anthem at minor league baseball game" type of thing. She should have that song SO cold she could do it in her sleep.
Even worse than the lyric flub were the horrible runs she put in. Yes, she's normally great at runs. Far better than most. But those came over like bricks falling from a wall. They stunk. Lea Michele, in her song, wisely abstained from all of that crap (and Michele usually isn't one for restraint).