I thought I was seeing things before when I noticed a "ghost" ward in the bushes. Then I saw it happen a few more times. And finally I put 2+2 together.
I was watching my LOL replay for a game where I saw such a "ghost" ward. What happened was I was in the bushes beside the enemies ward (set by a sightstone I watched to see who dropped it). They placed 1 more than the limit and one of the previous ones disappeared (I assume as intended). However when the previous disappeared it was REVEALED to me on the enemy team.
How does this affect gameplay? Well for instance another game later I saw the same thing happen near the baron. Had I known what the heck that glitch was that I saw I would have pinged and told my team to retreat. They had it warded but someone tossed down another and that one had been the one to vanish.
TL;DR When you drop another ward past the limit of sightstone it will REVEAL the ward that is supposed to vanish (for about 1-2 seconds it seems). Looks odd but definitely gives it away if you're near it.