For example, think of all the times people cross the street and DON'T get hit by a bus. Or all the people who catch the train to work on time and don't think twice about it. Or the dogs who wander the neighborhood and find their way home, unharmed.
In your story, maybe a careless bus driver hits a kid on a bike. Maybe a man misses his bus and ends up witnessing a crime, which leads to all kids of trouble. Or a dog goes wandering and is found by a girl who, though she means well, prevents him from getting home.
As a writer, you're always looking for extreme situations where things go wrong for your characters. When you consider all the things that DON'T do wrong, you are, in a way, imagining all the things that COULD go wrong!
Homework: Just for a day, try to recognize the many things that DON'T go wrong: the pipes didn't burst during your shower, you didn't get attacked by birds, you didn't lose your iPod at the beach, and you got to the gas station before running out of gas. This should give you a long list of troublesome issues to throw at your characters some day!