Triscadecaphobia (3.5e Trait)
From D&D Wiki
You have an inexplicable, intense fear of the number 13.
Prerequisite: You must have an intelligence score of at least 15.
Benefit: You get a +1 luck bonus on all rolls not made with a d20.
Drawback: If you or a party member roll a natural 13 on a d20, you must make a DC 25 will save. If you fail, take a -5 luck penalty to all rolls for four rounds.
Roleplaying Ideas: A person is always on guard for what they fear. In this case a character may look to 'find' 13s in other numbers. (1136 has 13 in the middle, 39 is 13 times 3, 58 has 5+8=13, etc) Alternatively the character may have a favorite number that they consider good or calming.