This is why idempotency is defined through visible state on the server. What happens on the server doesn't matter, but a GET request should not change the results of any subsequent future requests, as visible to the clients. E.g. adding or modifying a post or a blog entry via a GET request would break the rules.

(Nothing stops you in practice from ignoring that requirement, but it can lead to ugly surprises, such as a crawler triggering a mass deletion because it crawled the /delete links)