I’m sorry. I can’t reproduce the problem.
For what its worth, I don’t recommend using that built in editor. Its probably fine for CSS but if you make a mistake with the PHP files you can take your site down, even to the point that you can’t get into wp-admin to correct the mistake. Plus, it encourages editing without making backups by people who don’t really know what they are doing. I’m all for ‘fingering things out’ as someone I knew once put it, but not on a live site and not without backups. In short, that editor is in my opinion a very, very bad idea.
But…
What theme are you using?
Have you edited it at all?
Did this start after you edited it?
What are the permissions on the files?