I'll get karma-bombed into oblivion for saying this, but I've found it disturbing that a programming culture so obsessed with "testing" has so many problems keeping their servers up and running...
Even Hacker News seems to have skipped making load testing part of the process. Unit testing won't help you with these types of problems, but load testing is a pain and big companies skip it too sometimes, usually to their detriment.
That's because Twitter's architecture was designed using the Bob Dole school of infrastructure...it can't stay up.