Posted this 2 months or so ago (now 3) but no one bothered and its still in the game.
Sightstone is unusable under very specific conditions.
To replicate:
Once you upgrade to a Ruby Sightstone upon using the UNDO feature your old regular Sightstone becomes completely unusable.
We are currently aware of this issue. Hopefully we will have a fix soon, but this is one of those pesky problems that would take a substantial amount of work to address (the thing that causes this issue prevents other, more serious bugs I believe) and the although this can be frustrating to have happen, there are other more serious bugs that are higher impact.
In the future if you can tag issues like this with [GAMEPLAY] in the title and follow the guildelines here I will be able to sort through bugs and get work towards getting them resolved faster.
This. 
all you have to do is move the sightstone to a different item slot.
I just ran into this bug yesterday. I almost shat my pants when we where fighting near baron and I could not get any vision as my sightstone was disabled. Luckily I yellow pinged my team the hell of of there.
