This question regards the main theme from the video game The Last of Us. The video I will be referencing is someone playing it here:
There is a little introductory phrase at the beginning. Then, from 0:07 to 0:20, he plays the same riff three times, with little variations here and there. Then, on the fourth riff, the first note sounds off, but it still sounds good. The fourth variation of that riff, starting at 0:20, sounds somewhat eerie. Why is this? What is it about that note that makes it have that effect?