When using $wpdb->prepare(..) you may got this error:
Warning: Missing argument 2 for wpdb::prepare()...
Typically It is because you called the prepare function with only the query string. The correct way to do is something like this:
$wpdb->prepare( "SELECT * FROM table WHERE id = %d", $id );
But what if the function is called by a plugin written by someone and you don't want to mess with the query. Here is a quick way to fix that: Add null as the second parameter:
$wpdb->prepare( "<some query>", null );