Other mistake: Spoiler... When Stu Cobbler (Martin Starr) is revealed as the murderer, he goes to Gia's apartment to kill Veronica. Upon entering, Cobb has to disable an alarm with a digital touch pad, requiring the use of his fingers. However, he uses the metal barrel of the gun.
Suggested correction: Touchscreens don't need human skin to work. Capacitive touch screens just need something conductive (that's how touchscreen styluses work), and metal is conductive (albeit likely scratching the screen). But you also get resistive touchscreens, which simply detect pressure, and anything works on them.