What i expected to happen:
"testfor @a[score_x=0]" CommandBlock would give a redstone output for a score-reseted player.
What in fact happened:
No redst. output as long as i dont directly set that player to 0
Steps to reproduce:
-Create A Comm.Block > "testfor @a[score_AnyScoreName=0]
-Create A Redstone-Clock to it
-Put comperator next to the Comm.Block
=No output until you set your AnyScoreName directly to 0

Partial resolution:
Downgrade to at least 14w08a

Long story:
(I've been playing in snapshots for a while in this particular world.)
And Suddenly i found out that command blocks that used to set new players that joined in (with no scoreboard records > score 0) to various numbers.
But i found out (this might be bug from a slightly older snapshot) that the command block detect a players score as 0 only if i directly set it to that. If the player is new on the server or if you just do /scoreboard players reset... you end up with no result - the command blocks simply dont pick it up as 0.
I checked for typos but i've been using these exact CB before just fine. I tried downgrading (gradualy) and the version that it started working on was 14w08a (or lower) where it worked again with NO problem.