Entities that have a scoreboard value lose it upon rebooting a single player world.
To test:
1.In single player world, spawn Slime
2. Then do: scoreboard objectives add loseScore dummy
3. scoreboard players set @e[type=Slime] loseScore 1
4. tellraw @a

{selector:"@e[score_loseScore_min=1]"}

(this should say Slime)
5. Exit to main menu
6. Do tellraw @a

{selector:"@e[score_loseScore_min=1]"}

again (this should say "invalid json" because there is no such entity with that score)

This seems to only affect 1.8.4 and not previous versions...