Well, it's rather well known that system files control the system; breaking them has the potential to cause system-wide problems; I thought that much was obvious.

I'd guess you inadvertently saved it as html, as notes supports text and html, and will switch to html without a confirmation dialog.

If you want to edit system files, I recommend getting a real text editor; if you don't know (or care to learn) vi, then joe (my pick) or nano from xterm, or pygtkedit or a couple others from X.