Triscadecaphobia (3.5e Trait)
From D&D Wiki
You have an inexplicable, intense fear of the number 13.
Benefit: Once per day you can get a +2 to a save throw.
Drawback: If you roll a natural 13, consider it an automatic fail.
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.