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Imagine if they had only posted 10,000 or 15,000 characters, and it just slowed the site down. How fast would it have been noticed? Hours? Days?

Good monitoring (which I expect the SO guys to have) would have triggered on a spike in 95% or 99% response times so probably almost as quickly

Actually we average less than 10% CPU on the web tier... so anything over that warrants investigation. Graph of CPU during today's issue at https://twitter.com/Nick_Craver/status/755793398544601088