On 18 Feb 2007 forked_at_fark posted a thread where she asked Fark to help identify a picture of a UFO. Farkers immediately noted the resemblance to a streetlight but this was denied by forked_at_fark.
Two dys later, in a follow-up thread, forked_at_fark admitted the photo was of a streetlight but insisted there was something in the sky that night, just not the object that was in the picture.
Since then "It's a streetlight!" has become a common response to any threads about UFOs.