The word idempotent, as used in this context in the specifications, is (pseudo)mathematical jargon…and should not be taken too seriously or literally here. The phrase “no lasting observable effect on the state of the world” isn’t of course very exact either, and isn’t really the same thing. Idempotent processing, as defined above, does not exclude fundamental changes, only that processing the same data twice has the same effect as processing it once.

re: diff b/w Methods GET and POST in HTML forms