Twitter will only allow you to post a certain message one time. That means if you've ever tweeted "hi there!" in the past, you can't tweet it again. Because of this, I typically write my test messages to append a DateTime.Now to guarantee unique text.
To help out, the downloadable source code has a project called LinqToTwitterDemos. This project contains examples of using all authorizers and much more. There's also demo code on the Releases page and the Samples page.