When you summon an item on the ground and then add NBT data to it (like CanPlaceOn) it will work correctly. Summoning a second item of the type at the same location and adding the same NBT data to it will cause the two items to stack correctly. If you repeat this again, the third item, or anymore afterwards will not stack and you will still only have 2 items. The others just disappear.

Step to reproduce:
1. Summon an item on the ground (ex: sapling)
2. Apply NBT data, like CanPlaceOn, to the item using /entitydata
3. Repeat steps 1 and 2 again at the same location
4. Do it a third time. This time the item will just disappear instead of