This seems to be the reason why when moving/selling items in your inventory quickly it will sometimes swap it with an item right next to it. Also for why there sometimes appears to be 2 circles on the ground instead of just one. And perhaps the most annoying part is when it seems to do a 2nd click on empty ground right after clicking on an item to pick it up or a mob name to attack.
Could be a bug, or just an issue dealing with latency. Either way, it can be pretty frustrating.