Entities that have a scoreboard value lose it upon rebooting a single player world.
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
(this should say Slime)
5. Exit to main menu
6. Do tellraw @a
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...