Accessing GET variable named 'error'

Your problem is that you’re using Reserved Terms. error is a reserved term, you can’t use it.

To get around this, you could intercept the request early, and do a redirect, changing the query parameters in the process. For example, handling a link get parameter:

add_action( 'wp' , 'reserved_term_intercept' );

function reserved_term_intercept(){
    global $wp_query;
    if( $wp_query->queried_object_id == 72 && isset( $_GET["link"] ) ){

        wp_redirect( get_permalink( 72 ) . "?newlink=" . urlencode( $_GET["link"] ) . "&nonce=" . wp_create_nonce( 'nonce-name' ) );
        exit;
    }
}

Then handle the new variable:

add_action( 'wp_head' , 'reserved_term_handling' );

function reserved_term_handling(){
    if( wp_verify_nonce( $_GET["nonce"], 'nonce-name' ) == TRUE && isset( $_GET["newlink" ) ){
        printf( '<a href="https://wordpress.stackexchange.com/questions/185304/%s">Hello World!</a>', esc_url( urldecode( $_GET["newlink"] ) ) );
    }
}

wp_head may not be the most appropriate hook to run the code on, probably init instead

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