23 6 801

23 6 801 was a sequence of numbers that appeared a few times in the Doctor's universe.

History
At some point prior to March 2005, the Ninth Doctor scrawled the sequence on a wall. Steve M. saw him, and later documented it on Doctor Who?. (PROSE: Have You Seen This Man?)

In 2007, joe robjam texted the sequence of numbers to Suzy Monroe during the filming of Ghostwatch. (GAME: Ghostwatch)

Behind the scenes
According to an official response to a survey about Doctor Who that the BBC released in 2005, the "23 6 801" number sequence seen on the Psalm board in Father's Day was a mistake, and became a bit of "misleading fun". However, further clarification about the sequence wasn't given, such as how it was a mistake, and was present in television episodes filmed prior to Father's Day, thus casting potential doubt on this explanation. Adding to the confusion, when the WhoSpy production photographs were added to the gallery for Father's Day on the BBC's website, the caption suggested looking up Psalm 801, which no real world Psalm seems to correspond to.

As acknowledged by the survey response, there were many theories about "23 6 801": that the numbers corresponded to the 801st second of serial six (Mindwarp part two) of The Trial of a Time Lord (season 23), where Peri objected to the screams of animals; that it spelled Bad Wolf in latin; that it tied into the Bad Wolf story arc; and that it had something to do with the Daleks.

The sequence was seen in the following sources, both in- and out-of-universe:
 * Five instalments of WhoSpy
 * Mentioned in Have You Seen This Man?
 * Mentioned in Ghostwatch
 * And if the ALT code for "episode 13's" image on the Bad Wolf website is inspected, the sequence appears yet again.