Topic: Send appropiate header information on some instances and...
i.e header('HTTP/1.1 400 Bad Request') for non-existant topics and header('HTTP/1.1 403 Forbidden') for unauthorized actions.
I suggest you read this: http://karma.nucleuscms.org/item/27
Oh and I don't know if 1.3 fixes this, but BBCode similar to [ b ][ i ]foo[ /b ]bar[ /i ] creates invalid markup.