Other mistake: When Johnny and Mark save Denny from Chris R., they claim they are taking Chris R. to the police station. However, they re-appear in the scene almost immediately after (within 2 minutes of screentime, which is meant to be in real-time), with their having been obviously nowhere near enough time for them to have taken Chris R. to the police and done the appropriate interviews/paperwork.
Other mistake: When Johnny first goes into the flower shop to buy roses for Lisa, the clerk says she didn't know it was him. As he is leaving, she says he is her favorite customer. If he's her favorite customer, then there's no way she couldn't have known it was him, especially given how unusual he looks and sounds.
Add timeSerious B