Back in 2009 I came across a rendition of Amazing Grace sung by a gospel-choir-like group.
The second verse is sung a cappella by the bass in the group.
In the line
"How precious did that grace appear"
he drops an octave and sings really, really low.
I lost my mp3 copy of the song when a hard drive crashed in 2010 and I have been unable to find it ever since.
After some more digging I found this video. I'm pretty sure this is the person (Wes McKinzie) that sang verse 2 and it might even be the same group as well. The mp3 I had was a studio recording though (if I remember correctly).