There are several superstitions floating around our culture. They’re not always believed: “Step on a crack; break your mother’s back.” Others are more pervasive, like “knock on wood.”
How many people remember where that one comes from? A long time ago, when the more common belief held spirits were everywhere, people wanted to be thankful when good luck hit them and remember the tree spirits so they wouldn’t take their luck away.
Is there a good way to work in irrational beliefs into a character? Sometimes it’s difficult to see where they work into our own personalities, and making them work into fictional characters is just as dicey. Be careful, be consistent, and be sure you find the right motivation.