When a item is summoned with the PickupDelay tag set to 32767, any matching items that can stack up to the stack limit will join the stack and be unable to be picked up again.
Environment
windows vista 32-bit service pack 2, java version 7 update 51, 1GB memory, Intel Centrino duo, NVIDIA Quadro NVS 110M graphics card.
This problem is bigger than just the PickupDelay tag. I have created another report (https://bugs.mojang.com/browse/MC-47596) to show my findings