The attack on the fratboys in the first movie. Even worse when you wonder how many other times they have done it.
David kills one of them by biting him not in the neck, but through his skull.
Accoreding to the spinoff comic, Shane Powers, the Big Bad of the second film, was present and the attack was his Start of Darkness.
Thorn attacking Lucy even though he's been friendly with her before. Being Max's Hell Hound aside, it is a pretty realistic situation that can invoke primal fears to those that may have had been attacked by dogs.