Too Long; Didn't Read
There are multiple possibilities of what could go wrong when you get hit by traffic: CPU, RAM, storage performance and bandwidth limits. The most important thing is that if you don't know what's happening inside your system and exactly where it's happening, you can't fix it. Prepare to scale at 60-80% of maximum load. When you reach 80% of your resource limits, you should start scaling. You should act fast, because you’ll be losing your users and make more mistakes when you're in a hurry.