A character writes something down on a piece of paper that causes an extreme reaction to those who read it. Knowledge of what was written is withheld from the viewer for comedic effect and because, for example, the funniest joke in the world would be both above the abilities of the writer and an impossibility as poems/jokes are subjective. Crona's poem had the same effect on multiple people with different personalities.
(I am positive I have seen this trope elsewhere)
Monty Python - The Funniest Joke In The World (http://www.youtube.com/watch?v=8gpjk_MaCGM)
Soul Eater - Crona's Poem (http://www.youtube.com/watch?v=a_3631USjqw)