Believe it or not, there's a fair amount of people who still use Twitter by SMS. Which is fine, except that sometimes they don't realize what's going on when they get incoming texts.
Twitter began as a mass texting tool back at a time before most people had smartphones. You can still use Twitter by SMS and get notifications sent to you by text. There are still plenty of people who happily tweet this way, in the year 2012.
However, some people are confused when they start getting the texts from Twitter, and think it's someone harassing them. This is one of the most enjoyable things to see on Twitter — people raging out against the anonymous machine of texts, these Don Quixotes against their sea of SMS windmills.