Warning: Illegal offset

I was getting a similar error message when I updated Contact Form 7 to version 3.4.2. Replacing the add_shortcode function in wp-content/plugins/contact-form-7/includes/shortcodes.php from this:

function add_shortcode( $tag, $func, $has_name = false ) {
    if ( is_callable( $func ) )
        $this->shortcode_tags[$tag] = array(
            'function' => $func,
            'has_name' => (boolean) $has_name );
}

…to this:

function add_shortcode( $tag, $func, $has_name = false ) {
    if ( ! is_callable( $func ) )
        return;

    $tags = array_filter( array_unique( (array) $tag ) );

    foreach ( $tags as $tag ) {
        $this->shortcode_tags[$tag] = array(
            'function' => $func,
            'has_name' => (boolean) $has_name );
    }
}

…seemed to work for me. I think it has something to do with a change in PHP 5.4 and how it handles $this.