You run into all sorts of bugs you'd never even conceive of when things scale up, even basic frontend code

Some problem that would hit one customer every few years and go away on refresh for a local mom & pop website will irritate thousands of people a day on Google.com, and those people will band together on Facebook into a support group, and that support group will get media coverage

You try to look into the issue and you can't reproduce it at all, you just have to figure it out from tiny wisps of clues