When using the new scoreboard tags with a testfor command. The success can depend on the order in wich you specify the tags in a selector (@e).

Steps to reproduce:
-Make sure there are some entities besides the player in the world.
-/scoreboard players tag @p add test

Now this will correctly find the player:
-/testfor @e[tag=!blub,tag=test]

And this will incorrectly find all entities (I am not sure if it will find the player)
-/testfor @e[tag=test,tag=!blub]