The scene where Epps (Julianna Margulies) is in the emptied pool examining the bullet shells and the bullet holes in the wall: the shells and the holes indicate that the people who were murdered there were lined up against the wall and shot head on, leaving the casings on the bottom and the holes straight into the wall (as indicated by one bullet still lodged straight into the tile). In the flashback scene later in the film, however, Epps sees that the people murdered at the pool were shot above the water and then fell into the pool. While the casings could have just rolled into the water, any shots fired into the water at people still alive would have hit the wall at a downward angle, leaving marks unlike those we see in the scene. It's possible the holes could have been left by subsiquent visitors but the chronology indicates those holes were supposed to be from the initial massacre.