They are more common in the NFL than anyone cares to admit. You have to win some games that you are not supposed to. This is true because, another thing we try to avoid in reality, you will inevitably lose some games that you should win.
No one is going to act like the Super Bowl was won on an early October night in Pittsburgh. This is especially the case when you are fighting a shadow the size of your worldwide brand the way that the Dallas Cowboys are.
Ultimately the Cowboys won on Sunday night and did so against a Pittsburgh Steelers team that refused to take advantage of the opportunities they were being given.