Okay, Beyoncé didn't simply perform The Star Spangled Banner, she...let's just let Caity over at Gawker describe the performance:
"The initial moments of Beyoncé's inauguration performance were fraught with confusion, after she opened her mouth to sing the national anthem and everyone realized that God had replaced the artist formerly known as Beyoncé with an actual living angel whose voice was more heavenly than a thousand Hallelujah choruses."
"Then all of a sudden, in the middle of the song, something crazy happened: Beyoncé went rogue.
If you take a look above right around the 1:45 mark, you'll notice that Beyoncé casually removes her earpiece, mid-line, without missing a beat. It was later reported that she was plagued by audio problems throughout the song.
At this point in the performance, Beyoncé is, as they say, "singing blind." She no longer has any way to tell if she is singing on key, except for the fact that she is Beyoncé and "off-key" is the one note that exists beyond her range."
In the video above, it's about the 1:35 mark when the earpiece comes out.
Bravo, Beyoncé! Bravo, Caity!