When Beth Garrison and her daughter, Eve, discuss their concerns about Eve's younger brother, David, Eve turns her head toward Beth twice.
Danny is heard playing chromatic notes on an ordinary harmonica that has no chromatic slide.
The very idea that a swarm of rats would obey orders from a single individual rat is completely absurd. Rats might congregate in certain areas where they feel comfortable, but they do not act in a tribal manner. Indeed, they are more likely to turn on each other and fight to the death in times of extreme stress.
Toward the end of the film when the policemen and the firemen are attacking the rats in the sewers with flamethrowers and the rats are fighting back, it can be seen twice that the scenes of the flamethrower action have been filmed in reverse so that it appears that the rats are advancing - in fact, it can also be seen that the flames are going back into the flamethrowers.
Danny, with no formal music training, supposedly makes up "Ben's Song" on the piano, complete with an intricate major-minor progression containing minor sevenths and ninths, seventh-flat-five chords, etc.