Sometimes, events occur beyond our control, foresight, and budget. An unexpected incident—technological or otherwise—can wipe out all of our future projections. There are no magic theories or formulas to banish the capacity woes in these situations, but you might be able to lessen the pain.
Besides catastrophes—like a tornado destroying a datacenter—the biggest problem you are likely to face is too much traffic. Ironically, becoming more popular than you can handle could be the worst web operations nightmare that you have ever experienced. You might be fortunate enough to have a popular piece of content that is the target of links from all over the planet, or launch a new killer feature that draws more attention than you ever planned. This can be as exciting as having your name in lights, but you might not feel so fortunate at the time it’s all happening.
From a capacity point of view, you can’t do much instantaneously. If you are being hosted in a public cloud, it’s possible to add capacity relatively quickly depending on how it will be used—but this approach has limits. Adding servers can only solve the “I need more servers” problem. It can’t solve the more difficult architectural problems that can pop up when you least expect them.
It’s not uncommon to find that edge-use cases arise (probably more often than routine capacity issues!) ...