I think you're talking about the slight muted note around the 20 second mark?
It happens when you take your finger off the note too quick, so the string doesn't ring out, as the others are.
I'm surprised they didn't remove it/ record over it, though it does sound nicer, as if it's a proper acoustic-like song, as opposed to most of the others on the record...
Agreed. Although I like that riff when its played towards the end, at that break it just seems a bit too much, and makes the following verse sound a bit empty. Nice work on putting it in though :)