Koyuki's extra-long rendition of The Beatles' "I've Got A Feeling" Dying Breed's "Fifty Cent Wisdom"*
It's a common mistake among viewers, as the most prominent line in the song is indeed "I've got a feeling/in my head", but none of the other lyrics match the Beatles' song.
(not related in any way to Real Life artist 50 Cent), in which the other band members join him one by one until Beck is completely reassembled, if only for their gig at Greatful Sound.
In the Japanese original dub, he plays The Beatles' "I've Got a Feeling" instead.
Then there's the part near the end of the manga where the band performs "Devil's Way."
And the part in the final chapter of the manga where a typhoon causes all the electrical equipment at Beck's concert in Greatful Sound 9 to short-circuit mid-song, but the crowd just keeps on singing the song by heart.