A blurry picture my not be a ruined picture. Blur shows the presence of motion, which can give an artistic look to the image. The blur can help the viewer experience the action that was occurring at the time the image was taken.
Blur occurs when the shutter of the camera is open long enough that the objects in the image have moved while the shutter is still open. This is easy to do at night since the light is lower, which requires a longer shutter speed to get a proper exposure, all other things equal.