One thing I see on a fairly consistent basis in my Twitter stream is people who’s twitter accounts have been hacked or compromised.

Usually you get a DM (direct message) from someone with the following tweets

LOL…I am laughing so hard at this pic of me my friend found link added

Your naked pic is posted here
Hi this user is posting nasty rumors about you…
did you see this crazy tweet about you? l
Hey someone is making horrible rumors about you…
Are these rumors about you true?

In all these cases the message includes a link. If you click on the link your twitter account will almost certainly become compromised.  The best action to take is to send a direct message back to the person advising them that they have been hacked.

You should then change your password and check what apps are using your Twitter account.  Blogger Redhead Riter (really!) has an excellent step by step guide on how to get your Twitter account back.