Huh, I said to myself. That replacement file is a lot smaller than the original file. How strange. I only deleted the 'office closed' text. It shouldn't be that much smaller, should it? Ah, well, maybe it is. I'm sure I can't screw this up that much. I've still got my editor open; I can just undo as far back as I need to to restore the original version if it comes to that.
And THAT'S when I broke the webpage.
(The original file that I'd opened hadn't finished downloading off the server, so the end of it was truncated when I opened it to edit... DOH.)