Factual error: A fire breaks out in the hospital due to one of Abby's victims being exposed to sunlight as she's turning into a vampire, destroying that vampire and a burning a nurse to death. Modern hospitals have fire sprinklers, fire extinguishers, etc. and sensors/alarms that the fire would have activated.
Continuity mistake: When Abby slaughters the bullies, we see most of the scene from Owen's underwater point of view. First, a decapitated head floats down, leaving a trail of blood. Then, more dismembered bodies splash into the water. We get the sense that the pool water around Owen should be pretty bloody. Yet, when he surfaces, the water around him is completely clear, with no evidence of any blood or bodies.
Chosen answer: The thing that zooms across the water appears to be Abby herself - upon breaking the window, she beheads the boy holding Owen under the water almost immediately after she enters the pool room; her incredible speed makes it possible that she moved rapidly over the water without sinking below it.
Purple_Girl
Its one of the bullies upside down you can see if you pause at the right time or slow it down.