With the foreach commented out, the code runs without errors.

If this line is uncommented:

           foreach ($sale->getInvoices() as $invoice) { }

Then the code breaks with this error.

Notice: Undefined index: 00000000583b34730000000060d36b73 in .../
vendor/doctrine/lib/Doctrine/ORM/UnitOfWork.php on line 2053

= = = = = = = = = = = = = = = = =

public function indexAction()
{
        $em = $this->get('doctrine.orm.entity_manager');

        $q = $em->createQuery("select s from CoreBundle:Sale s");
        $results = $q->getResult();

    $count = 0;
    foreach ($results as $sale) {
        $sale->setValue(rand());

        // FOREACH COMMENTED OUT
        // foreach ($sale->getInvoices() as $invoice) { }

        if (0 == $count++%50) {
            $em->flush();
            $em->clear();
        }
    }
}