When playing as Vi 
just about every 3rd game or so I run into a problem mid game. I will engage in a gank and use my "Q" to get in and try to use "Q" again to leave but this time it says "you cannon use the spell". this will last the duration of the game. Has anyone else encountered this? Im still not sure when it is triggered, my latest thought is that when I am stunned/silenced mid dash it breaks it?
I've had this happen to me as well, it only seems to happen when her Q is interrupted while it's charging (so our situations are slightly different). Either way I've found that casting Q on yourself (alt+Q) fixes the problem and you are then able to use your Q like normal after that.
My Q for Varus was doing the same thing. Stopped working mid game and I had emailed Riot and was told to post here as well. I think it's because I was silenced/stunned and from then forward for the rest of the game my Q was not working. I tried reassigning the key to anything else as well as restoring it to default and nothing changed the bug. Hopefully this is resolved.
This happens to me on sion. Can't use his q or r
This happened to me with 
as well. It useally happened when i was charging a shot and got knocked up/killed.
Please riot fix these Q bugs.
As a side note to others faceing this problem I found a way of fixing the Q bug temp. Log out and relog back in. After you log back in Q will work fine, atleast for who has to have his Q.
My Q also stopped working around minute 15 and could not get to fix it, i tried rebinding my skills re logging nothing. I was playing with Kat and owning till this bug happened. Please fix this bug, I got reported for the negative attitude, please remove this report since it was not my fault.
I've had this happen to me on vi as well. I had to reload the game to get it to work.
