What happens
If you spawn a mob, i.e Villager, then kill it, then spawn in more of that same mob, and try to run command on the entity, or test for the entity fails, like the entity does not exist.

How to recreate the bug:
Get a spawn egg, Villager, then kill one or more of the villagers (these entity's can be successfully executed commands on), then try to run a execute @e[type=Villager] or testfor @e[type=Villager]

What will happen:
and it will error with "That entity cannot be found", even though they are in the world. The way to temporary fix this is by saving a exiting out of the world then re-entering and it will work find

What should happen:
If you spawn in the villagers, then kill them, and span in more you should be able to run commands such as execute and testfor @e and find then when the're in the world.