our code that initiates the sequence

function spf_rpx_signin_user($auth_info)
{
  $identifier = $auth_info['profile']['identifier'];
  $current_user = wp_get_current_user();
  $wpuid = spf_rpx_get_wpuid_by_identifier($identifier);

  # if we don't have the identifier mapped to wp user, create a new one
  if (!$wpuid)
  {
    $wpuid = spf_rpx_create_wp_user($auth_info);
  }

  # sign the user in
  wp_set_auth_cookie($wpuid, true, false);
  wp_set_current_user($wpuid);

  # redirect them back to the page they were originally on
  wp_redirect($_GET['goback']);
  die();
}

namely this code

wp_set_current_user($wpuid);)

after deteriming the user does exist...

the top part of the backtrace...

array(15) { [0]=>  array(4) { ["file"]=>  string(52) "C:\wamp\www\wordpress-dev\wp-includes\formatting.php" ["line"]=>  int(738) ["function"]=>  string(17) "wp_strip_all_tags" ["args"]=>  array(1) { [0]=>  object(WP_Error)#176 (2) { ["errors"]=>  array(1) { ["existing_user_email"]=>  array(1) { [0]=>  string(41) "This email address is already registered." } } ["error_data"]=>  array(0) { } } } } [1]=>  array(4) { ["file"]=>  string(51) "C:\wamp\www\wordpress-dev\wp-includes\pluggable.php" ["line"]=>  int(200) ["function"]=>  string(13) "sanitize_user" ["args"]=>  array(1) { [0]=>  object(WP_Error)#176 (2) { ["errors"]=>  array(1) { ["existing_user_email"]=>  array(1) { [0]=>  string(41) "This email address is already registered." } } ["error_data"]=>  array(0) { } } } } [2]=>  array(4) { ["file"]=>  string(51) "C:\wamp\www\wordpress-dev\wp-includes\pluggable.php" ["line"]=>  int(230) ["function"]=>  string(11) "get_user_by" ["args"]=>  array(2) { [0]=>  &string(5) "login" [1]=>  object(WP_Error)#176 (2) { ["errors"]=>  array(1) { ["existing_user_email"]=>  array(1) { [0]=>  string(41) "This email address is already registered." } } ["error_data"]=>  array(0) { } } } } [3]=>  array(4) { ["file"]=>  string(54) "C:\wamp\www\wordpress-dev\wp-includes\capabilities.php" ["line"]=>  int(489) ["function"]=>  string(19) "get_userdatabylogin" ["args"]=>  array(1) { [0]=>  object(WP_Error)#176 (2) { ["errors"]=>  array(1) { ["existing_user_email"]=>  array(1) { [0]=>  string(41) "This email address is already registered." } } ["error_data"]=>  array(0) { } } } } [4]=>  array(7) { ["file"]=>  string(51) "C:\wamp\www\wordpress-dev\wp-includes\pluggable.php" ["line"]=>  int(51) ["function"]=>  string(7) "WP_User" ["class"]=>  string(7) "WP_User" ["object"]=>  object(WP_User)#69 (10) { ["data"]=>  NULL ["ID"]=>  int(0) ["id"]=>  int(0) ["caps"]=>  array(0) { } ["cap_key"]=>  NULL ["roles"]=>  array(0) { } ["allcaps"]=>  array(0) { } ["first_name"]=>  string(0) "" ["last_name"]=>  string(0) "" ["filter"]=>  NULL } ["type"]=>  string(2) "->" ["args"]=>  array(2) { [0]=>  object(WP_Error)#176 (2) { ["errors"]=>  array(1) { ["existing_user_email"]=>  array(1) { [0]=>  string(41) "This email address is already registered." } } ["error_data"]=>  array(0) { } } [1]=>  &string(0) "" } } [5]=>  array(4) { ["file"]=>  string(76) "C:\wamp\www\wordpress-dev\wp-content\plugins\simple-forum\library\sf-rpx.php" ["line"]=>  int(211) ["function"]=>  string(19) "wp_set_current_user" ["args"]=>  array(1) { [0]=>  object(WP_Error)#176 (2) { ["errors"]=>  array(1) { ["existing_user_email"]=>  array(1) { [0]=>  string(41) "This email address is already registered." } } ["error_data"]=>  array(0) { } } } }