When copying/pasting text from other applications (ie. MS Word, Notepad, etc.), it is very important to clean up the formatting that is brought into the HTML editor with that text.  You will likely not even see that the text you copy & paste has extraneous formatting and code associated with it, nonetheless it does and you should perform three basic commands from the editor menu to remove this code, to avoid conflict with code generated by the HTML editor.  (Note:  if you want to see a sample of some of this code, simply select the "<>" command "Switch between html/text" and you'll see the endless lines of code that are inserted amongst your text, no matter how clean you think that text was when you pasted it.

The three basic commands are as follows:

"Cleanup Formatting"
"Cleanup Font Tags"
"Cleanup Word"

Execute these commands by clicking their corresponding icons from the HTML editor, represented by an erasor, a paint brush, and a blue "W".  This will strip out all extraneous code and give you a clean start for any formatting you wish to do.  Keep in mind that any formatting you do will override changes in your website's Stylesheet however.