Apply Filters Causing a 500 Internal Server Error

Please try to replace your snippet with the following. The global $post isn’t needed, when it’s outside a function or method context and inside the loop, as then $post is already global and get_post_type()s default false arg with be replaced with $post inside the function.

if ( 'customt' !== get_post_type() )
    return print apply_filters( 'the_content', $post->post_content );

Also make sure, that you’re not calling it on some too early filter, like plugins_loaded or init, as the global $post object wouldn’t be setup up.