Paste as Plain Text

It's important to ALWAYS use the Paste as Plain Text tool when pasting in text from another source. To do this, clic the little clipboard icon in the text formatting toolbar, paste in your text and click the ok button. This will strip out all the formatting that might come along with the text so it won't look inconsistent with your site design or cause errors.

Learn more: 
Using the Text Block
Fixing Text Formatting