Affects Version/s: Minecraft 14w20b
Fix Version/s: None
Found the reason:
Summon multiple mobs with the same UUID
the game thinks it killed all mobs (even the ones with same UUID) and you get also the same count of reports like there are mobs
When you hover them they have both the same UUID
BUT actually only one of them was killed.
So you get the error message: "That entity cannot be found"