Anything you believe in can happen, but mostly in coincidences.
For instance, I was driving down town with my sister, and a black cat walked across the street in front of us. She made me go around the block and not drive where the cat had walked.
I told her, "What if the cat had walked across the street ten minutes earlier, would it still be bad luck, if we didn't see it"? She didn't know what to say. I told her a cat is a cat and nothing else.
But a friend of mine had some experience with an owl. She was living on a farm, with her grand parents. She got real sick, and was almost to die. Her granddad was walking outside the house and he saw an owl sitting on a fence post just outside the house.
He went inside and told Grandma to go out and chase the owl away, to curse it and yell at it, for it wasn't an ordinary owl. It was after the teenage girl.
Grandma went out and cussed the owl out, using words she normally wouldn't think of using and the owl finally flew off. The girl then started coughing and vomiting, and she coughed up a bunch of hair, a hairball. And then she got better.
Grandpa said that the owl was a curse or something, put on the girl, but it was chased away and the curse was broken . He had dealt in some minor witchcraft himself, in his younger days.
So, it all depends on the circumstances. I don't believe in superstition, but I do in spirits.