Too Long; Didn't Read
A scalability problem involves at least two-dimensional thinking: load metric and a time-period. The web is huge, and it's getting bigger, and more people are doing more things. Only a handful out of every 100 developers nowadays is really into building things that can scale. In an e-commerce website, for example, the system may scale well to serve 10,000 people shopping at the same time across a thousand-item catalog. But what happens if thousands try to decrement the same value in a short period?