Data Center Crash from Faulty Loop

When working on social network ads at Google (remember Myspace?), I wrote some C++ code that looked something like this:

for (int i = 0; i < user->interests->length(); i++) {
    for (int j = 0; j < user->interests(i)->keywords.length(); j++) {
        keywords->add(user->interests(i)->keywords(i)) 
    }
}

Readers who are programmers probably see the mistake: The last argument should be j not i. My unit tests didn’t catch the mistake, nor did my reviewer. After going through the launch process, my code was pushed late one night—and promptly crashed all the computers in a data center.