This commonly happens when you cut-and-paste text from a website, into Word or a similar program, and into the expert page editor. The programs "remember" the code and try to stick it in your expert page.
The best thing to do is to open Notepad, paste the text there, and save it as a plain text file (.txt). Now, open the expert page editor. Delete everything in the field giving you a a problem (press ctrl-a to select it all, then "delete" to delete it). Then, paste in the plain text from Notepad.