They explained to me that they'd intentionally slam the production website with external traffic a couple of times per year, at a scheduled time in the middle of the night. Like basically an order of magnitude greater than they'd every received in real life, just to try to find the breaking point. The production website would usually go down for a bit, but this was vastly better than the website going down when actual real users are trying to sign up for the Boston Marathon.

Slack probably should've anticipated this surge in traffic after the holidays, and if might have been able to run some better simulations and fire drills before it occurred.