tip: "uchiha sasuke/uzumaki naruto" angst kudos>10
Don't know whether this is still a problem, but as far as I'm aware, fanfiction.net has an active policy of blocking web crawlers/spiders/bots (other than the ones they let in themselves). When their system notices a massive amount of requests in a short time coming from a limited set of IPs, it just blocks those IPs. I don't know whether they'd allow AO3 in as an exception to that policy, since its one of the measures they take against their servers being overloaded.
Posted Wed 18