I have at least one PDF where when loaded by Zend_PDF, Zend_Memory_Container_Locked objects are pulled into the Document Type Dictionary properties array.

Array ( ... [SaveStreams] => Array ( [Q] => Zend_Memory_Container_Locked Object ( [value] => Q )

        [q] => Zend_Memory_Container_Locked Object
            (
                [value] =>  q 
            )

    )

[Changes] => Array
    (
        [0] => Array
            (
                [Creator] => SPDF
                [Producer] => SPDF
                [ModDate] => D:20060401000353-05'00'
                [CreationDate] => D:20060401000353-05'00'
            )

        [1] => Array
            (
                [Version] => 3.0, SPDF Build Number 1085 for Linux 7, Application Build Date: May 15 2003
                [ModDate] => D:20060401050353-00'00'
                [Product] => AppendPro
                [Vendor] => Appligent
            )

    )

... )

If you then modify the properties array, saving crashes with a fatal error.