Okay looking at the code there are two possible ways you could be getting the error.
PHP Code:
if($id == "" or !is_numeric($id)){
$article_title = $err_idnoexist;
$article_content = $err_idnoexist_text;
}
PHP Code:
else {
// Adoptable does not exist, show an error.
$article_title = $err_idnoexist;
$article_content = $err_idnoexist_text;
} // End the else for if $id == $aid
(Quite frankly the second one is VERY redundant but oh well)
As far as the code goes, the second issue would be caused by a bad mysql statement, but it looks as though it would work just fine and I'm assuming other mysql statements on your site are working...?
If the case is the first one causing the problem, it would mean the $_GET($id) is not working properly and you'd have to post the adopt.php code for me to see if there's an error though. (sorry troubleshooting can be annoying)