Some complex objects won't be correctly stored into the session. This is probably a problem with serialize/unserialize. It happens with objects structure containing some references. The code below builds the simpliest structure I could find that pinpouts the problem.
With more complex structure, you can have all sorts of strange behaviour, including crashing PHP, but the behaviours vary from one PHP installation to another (even with the same version and compile flags).

<?php
/*
View this script with a browser and press reload
It will probably display "-->137645276" instead of "-->Object" 
You can view it at http://www.generasound.com/test/bug.php
*/
session_start();
if (!isset($form))
{
  $form = new stdClass();
  $submit = new stdClass();

  $upperif = new stdClass();
  $upperif->Child = &$submit;
  $upperif->Layout = new stdClass();
  $upperif->Layout->Child = new stdClass();
  $upperif->Layout->Child->Interface = &$submit;

  $lowerif = new stdClass();

  $form->Children[0] = &$upperif;
  $form->Children[1] = &$lowerif;
  $form->Layout = new stdClass();
  $form->Layout->Children[1] = new stdClass();
  $form->Layout->Children[1]->Interface = &$lowerif;
  echo "-->".$form->Layout->Children[1]->Interface."<BR>";
  session_register("form");
}
else
{
  echo "-->".$form->Layout->Children[1]->Interface."<BR>";
}
?>