Thirteen

Because of Jick's supposed triskaidekaphobic tendencies, various elements of the kingdom avoid the number thirteen since it is, apparently, unlucky.

Things that avoid the number thirteen:

• Adventure IDs
• Choice IDs
• Effect IDs
• Familiar IDs
• Item IDs
• Outfit IDs
• Most Skill IDs
• Trophy IDs

Things that use the number thirteen:

• Mr Skullhead's player ID
• Item #13
• 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 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 produces 13 different items.
• The first phase of Spooky Advent Calendar was 13 sections long.
• Tiny Plastic Series 1 numbers
• Also, see Friday the 13th for some more superstitious fun.
• The levels at which quests opened up has stayed at 11 for a long time; the expected NS13 update should change that.
• The Traveling Trader talks in ROT-13.
• The encoder ring codes/decodes in ROT-13.
• A plastic pumpkin bucket increases item drops by 13%.

Other unlucky happenings:

• At one time, the item number 666 was avoided for superstitious reasons also. Nevertheless, BoozerBear managed to hack 13 of them from the item database and kept them in his inventory, until one day as a meatsink, Jick implemented 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.