I sketched out the design and it looked clean on the whiteboard, and then someone asked what happens when the traffic is ten times bigger. I didn't have a good answer. Every choice I make trades one thing for another, speed for cost, simple for flexible, and I can never tell if I've picked right until it's live and straining. I can build the thing, I just can't be sure it'll hold when it matters.
More people experience this than they realize.
You can design a system, but you can't tell if you've made the right tradeoffs until it's already under load.
If this sounds familiar, the Library can help you find the bigger picture.