This a really good question – my suggestion is to use transients. For instance, in your validation callback:
wpse51669_validation_cb($settings){
//Perform validation checks
if( $valid ){
//If settings validate
return $validate_settings;
}
//Otherwise add settings error
add_settings_error('my-plug-in-settings','error-with-xyz', 'I fell over','error');
//And add the failed settings to a transient
set_transient( 'my-plug-in-settings-invalid', $settings, 60);
return false;
}
Then on your settings page, if the options have been validated – check for the transient, and use the ‘failed’ settings, if they are found:
if( isset($_GET['settings-updated']) && $_GET['settings-updated'] && get_transient('my-plug-in-settings-invalid') ){
$options_to_display = get_transient('my-plug-in-settings-invalid');
delete_transient('my-plug-in-settings-invalid');
}else{
$options_to_display = get_option('my-plug-in-settings');
}