When Gilkrest calls the police, he's using a phone with a rotary dial. It's assumed that he's calling 911, but on the soundtrack the length of the sound of all three instances of the dial returning to its "rest" position is exactly the same; the sound from dialing the "9" should be much longer than for the "1"s.
(at around 1h 5 mins) A letter on a hall stand incorrectly spells Gilkrest as Gilkrist.