Brad started to play the beginning of "Valentine's Day" in the wrong key, which prompted Chester to follow along and sing higher than usual to match the guitar part. Brad walked over to Chester during the first verse, likely noticing the mistake, and Chester pointed at him. Then, when the rest of the instruments started, Brad and Chester both changed back to the normal key for the rest of the song.
Sources:
Time: 90:51 mins
Format: mp3 / 320 kbit/s
Comments: The mess up on "Valentine's Day" was edited and fixed with a portion from the show the day before in France.