Continuity mistake: When Spider-Man is launched against the library, when he lies on the floor the type of books next to him change between shots.
Continuity mistake: Just when Peter tries the web launcher the tables and chairs on the street change number and position through shots: They are arranged parallel to each other and have the following amount of chairs: First table has 3 chairs, second 2, third 1 and fourth 2. In the close-up, all tables have 2 chairs; the first three tables are parallel, yet a fourth one is behind the third table. Peter also knocks down a table or chair which appears standing in the final shot.
Answer: For fun. It's common to prank people who are sleeping in public to see if they'd wake up.
lionhead