After defeating the Mayor, Dog Man ends up taking her to "Mayor Jail". At first glance, this might seem like a cute gag...until you wonder why they need a jail specifically for mayors. How many Corrupt Bureaucrats has this city had?
The intro chapter of For Whom the Ball Rolls reveals that Harold helped co-write this story. Then you see Peteys character arc of this story, which specifically is about his father, who abandoned him when he was young, coming back into his life and treating him horribly. This is definitely a Fridge Tearjerker when you know Harolds backstory as revealed in the 9th Captain Underpants book, but this quickly turns to Fridge Horror when you reach the scene of Peteys dad accidentally smashing dinner plates all over the place as Petey and Lil Petey cower in fear. Did Harold just subtly imply that his father was physically abusive?