Oh! I forget what that was called, but I remember reading about it on some web page.... it happens in TextEdit in certain fonts, too, and I would assume so with other Cocoa apps.
Yeah, that's something to do with unicode. Check out http://homepage.mac.com/earthlingsoft and get UnicodeChecker. It's great for finding characters, such as Devanagri (since Apple still hasn't developed half of the text inputs they had in OS 9).