Twitter is not a private chat room. It is a publishing platform, and your tweets are publicly viewable, even to people who don't follow you. But because Twitter can often feel like an SMS chat with your mates, it's easy to forget that everything you write (except in a direct message, of course) is both public and permanent.
Think you could get away with posting something rude or libellous if you quickly delete it afterwards? Bad luck. Because not only does that tweet still appear in searches, but there's now a tool called Tweleted that can recover your deleted tweets.
Tweleted has a "good" mode - to find tweets you've lost or deleted accidentally - and an "evil" mode - which "recovers embarrassing deleted tweets for fun or profit... an instant drama generator".
Ah ha, you say - but what if I block someone? They won't be able to read any of my tweets then, will they? Err, in a word: yes. Even if I block you, and we no longer see each other's updates, all you have to do is log out of Twitter and visit my Twitter page and you can see everything I've written. Don't be fooled into thinking that blocking on Twitter is like blocking on Facebook: it isn't.
In fact, don't be fooled into thinking Twitter is anything like Facebook. You might have grown accustomed to Facebook's "walled garden", where status updates and wall posts were only visible to your mates. But let me say it again: everything you post on Twitter is permanent and public.