To reproduce: 
1. Open DemoApplication => RichTextBox example
2. Copy this string of text: http://www.telerik.com/account/get-started/developer-tools-installation.aspx?p=o5NOHjJzPGGf%2fYOZH6RwzGjDrhxlFgmsIPLPsdVgHvfLcqjjCRQey%2flTyABKZAWf&utm_source=msi
3. Inserted it at the top of the RichTextBox demo, before the "TelerikUIforWinforms" logo at the top. He then highlighted it and Underlined the text, causing the text to get completely jumbled. It ends up getting fixed by selecting Bold. So toggling underline causes it to get jumbled. It gets fixed by toggling another setting, like bold. 
You can see the attached image. 

Resolution: 
This issue is addressed in the new version of the control - RadRichTextEditor. Please use the new control instead the RadRichTextBox.