The Pronamic iDEAL plugin gives the following message when WP debug mode is enabled :
Deprecated: Assigning the return value of new by reference is deprecated in /.../wp-content/plugins/pronamic-ideal/classes/Pronamic/Jigoshop/IDeal/AddOn.php on line 65
Line 65 is the following:
$order = &new jigoshop_order( (int) $id );
Strangely, this message only occurred after I upgraded to WP 3.6, even though that should not be an issue.