Save_post not being hooked properly?

OK, major headscratching moment. I have:

add_action('save_post', array(&$my_plugin_instance, 'my_plugin_save'));

And then in the main plugin class:

function my_plugin_save( $post_id ) {
	echo "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa";
}

The ONLY time “aaaaaaaaaaaaa” gets echoed is when I choose New Post or New Page. Once I click Publish or Update, my_plugin_save doesn’t get hooked anymore.

Totally mystified… Anyone help?

Mark