Thirteen

Because Jick thinks that triskaidekaphobia is funny, various elements of the kingdom avoid the number thirteen.

Things that avoid the number thirteen:

Things that use the number thirteen:

Mr Skullhead's player ID is 13.
The Item #13 is an obvious reference to the number.
King Ralph XI was originally named King Ralph XIII; however, at the suggestion of the player Lord Ingo, Jick agreed that it would be ridiculous and not even funny if it were XI instead.
The 2005 Advent Calendar produced aspirin with the message, "They can't all be winners, kid," on its thirteenth day, supposedly for the same reasons as noted above.
Spooky Toymaking used to produce 13 different items.
The first phase of the Spooky Advent Calendar was 13 sections long.
The Tiny Plastic series 1 and 2 both have a 13th member.
See Friday the 13th for some more superstitious fun.
The Naughty Sorceress is unlocked at level 13.
The Traveling Trader talks in ROT-13.
The encoder ring codes/decodes in ROT-13.
The plastic pumpkin bucket increases item drops by 13%.
There are 13 items to collect in the quest Going Postal.
The cursed piece of thirteen is another obvious reference to the number.
The Dirty Laundry trophy is given by the Trophy Maker for having 13 spooky pirate skeletons in one's closet.
The puzzle in A Volcanic Cave is a 13x13 grid.
There are thirteen different colors of Slime Potions.

Other unlucky happenings:

At one time, the item number 666 was also avoided for superstitious reasons. Nevertheless, BoozerBear managed to hack 13 of them from the item database and keep them in his inventory, until one day, as a meatsink, Jick implemented the steaming evil as item number 666.
Uncle Crimbo's player ID is 666, a foreboding sign of his eventual fate.
Breaking an antique hand mirror gives you Bad Luck.