1. A post that was published, but then set to "Unpublished" (and even Private!) is still accessible by URL.
2. If functions.php contains incorrect PHP code, the editing page is no longer accessible because it compiles the code to load the page (this makes sense, since every page must include functions.php, but it would be nice if WP somehow detected bad code.
(So far, the only way to fix this is to download functions.php with a FTP client, fix it, and reupload)
3. I'm not sure if this is temporary, bug WP redirected me to http:/#form or something like that when I tried to log in.