I was doing some interviews about systems used at universities to book classes for semester. These systems are famously poor at handling the huge spike every half year when thousands, or tens of thousands students are let in all at once and try to book all their classes.
What I didn’t realize, and learned only today, is this: Since each operation is a lottery whether or not the response arrives, many people actually open browsers on all devices available to them and even ask their friends and family to help them out… That right there multiplies your traffic and load and sends the problem to a death spiral 
Hard to say what portion of users does that, I hear it’s quite common, but the moral of the story for me is how easily a problem can blow totally out of proportion.
