In the book, a little girl asks to try on Hazel's cannula in the mall, and Hazel lets her. The girl's mom apologizes for the moment. In the movie, this scene was cut, but an alternate version was shot featuring a cameo from author John Green! This moment instead takes place in the airport before Amsterdam, and Green plays the girl's dad. The scene will be an extra on the DVD.
Source: Twitter user realjohngreen