This also happens to me. Can be reproduced by doing this: Press shift a few times in a row, then hold it. It looks like your character is blocking but you can move fast, meaning you aren't blocking. The odd thing is you can use the At Will powers that you're only supposed to be able to use while blocking.
The issue is that our character goes into "block mode" and can use the at-will powers that you can only use while blocking, however we're not blocking and we can move very fast (can't move fast when actually blocking).
Well, they tried that with GF and it broke our ability to Guard http://nw-forum.perfectworld.com/showthread.php?285582-GuardianFighter-Block-button-problems/page3