There's an octave and a half stepwise run in the first part of this song that looks like this:
All is well when it lands on A (the fifth of the D chord) for the downbeat of the last bar, but it's a bit of a bumpy journey to get there. The offending note is the D over the C chord on the strong third beat of the previous measure. Listen to this on the recording and you will hear how dissonant it sounds.
It certainly took some discipline to sing this whole passage correctly. How it might have been composed is an interesting question.