Don't walk under that ladder!
Frankly, this superstition is pretty practical. Who wants to be responsible for stumbling and knocking a carpenter off his perch? But one theory holds that this superstition arises from a Christian belief in the Holy Trinity: Since a ladder leaning against a wall forms a triangle, "breaking" that triangle was blasphemous. Then again, another popular theory is that a fear of walking under a ladder has to do with its resemblance to a medieval gallows. We're sticking with the safety-first explanation for this one.