Update: Proving the point detailed in this article, Pharrell’s “Happy” has reclaimed number one on the iTunes Single Sales chart. Katy Perry’s “Dark Horse (featuring Juicy J)” returned to number two, while John Legend’s “All of Me” has slipped to third.

In knocking Pharrell’s smash hit out of the number one position, a notorious iTunes glitch helped make another popular artist very “Happy.”

For the first time in his career, singer-songwriter John Legend is sitting at the top of the iTunes Single Sales Chart.

Responsible for the career milestone is red-hot single “All of Me.” Owing its popularity explosion to Legend’s live rendition at the Grammy Awards, the single reached the top of the iTunes single sales chart early Thursday.

In getting there, however, “All of Me” received some assistance from an iTunes issue that has recently affected artists like Katy Perry, Lorde and Aloe Blacc.

Because the iTunes sales chart treats the different versions of a song as separate products, it routinely stumbles when a single (or soundtrack, in this case) version of a song is replaced by an album version. Unable to carry over the previous version’s sales data, the chart treats the album version as a fresh track with a sales count of zero.

As a result, when iTunes replaced an earlier version of Pharrell’s “Happy” with the track from his newly confirmed “G I R L” album, it could no longer recognize the song as its top seller. Pharrell’s song would have to restart from the bottom.

With “Happy” temporarily out of the mix, Katy Perry’s “Dark Horse (featuring Juicy J)” was able to ascend from number two to number one. John Legend’s “All of Me,” meanwhile, climbed from three to two.

Still riding a firestorm of momentum, “All of Me” then leapfrogged the Perry song to reach the top of the iTunes mountain.

Insofar as “All of Me” was (and is) clearly on the rise, it very conceivably could have reached number one on its own merit. The iTunes issue did, however, forge an accelerated path to the top.

As of press time, “Dark Horse” remained behind in second place behind “All of Me.” Pharrell’s “Happy” had rebounded to three, while Jason DeRulo’s “Talk Dirty (featuring 2 Chainz)” and Bastille’s “Pompeii” followed in fourth and fifth, respectively.

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