Dying mobs can destroy items
When a mob that can collect item dies it first drops some loot, but still can collect items afterwards.
Those items won't get a chance to be dropped again and just vanish. This includes the mobs own (rare) drops!
Steps to reproduce:
1. Find a mob that can collect items
2. Kill it in a way that it will be in range of some items (or his own loot)
3. Notice how it collects the items (the "dead" mob holds them) and disappears
Video showing the process: http://www.youtube.com/watch?v=cSFhVL_ES3M