"If one reads the Megillah backwards, he has not fulfilled his obligation." (Gemara, Megillah 17a)
Why would one do such a strange thing?
The purpose of reading the Megillah is to bring to our attention that in all generations there are Hamans who plot our destruction. Miraculously, Hashem comes to our salvation thanks to our adherence to Torah and mitzvot.
"Reading the Megillah backwards," means thinking that the Purim narrative is a historical event with no contemporary relevance. One who takes such an attitude is missing the entire significance of Purim and the Megillah.