The fourth episode of the new season of Atlantis is now available for download … accidentally. Fans in the U.S. who headed over to iTunes to download the season premiere “Adrift” have instead discovered that they have purchased “Doppelganger,” set to premiere October 19 on SCI FI Channel.

The mix-up likely comes from the fact that “Doppelganger” was the first episode of the season to be filmed, and thus carries an internal production number of 401 at the studio.

The episode is listed on iTunes as “Adrift,” and can be found by searching for the title or looking in the new releases section. The main page for Stargate Atlantis has not yet been updated for Season Four, which premiered September 28 in the U.S.

There is no word from iTunes yet on when the error will be corrected, and whether viewers will be compensated with the correct episode.

The episode was posted on Saturday and, as of noon on Sunday, was still available for a $1.99 download price. Fans hoping to get an early look at “Doppelganger” should be warned, though: if the file is swapped for the real “Adrift” before you start your download, you’ll be purchasing “Adrift.”

New episodes of Stargate Atlantis continue every Friday at 10 p.m. Eastern/Pacific on SCI FI! Learn more about what is to come in GateWorld’s episode guide.

(Thanks to Sean, Boone, Matt, and Briangate78 for the tip!)