As you can see in this video, http://www.mediafire.com/download/prqtfpwdrcic83k/javaw+023.avi
When you have a potion in both hands and right-click, you throw the right potion and (only graphically) the left potion.
It looks like you were throwing repetedly right-left-right-left, but for real you only throw the potion in your right hand.
So this is just a minor graphical thing that isn't as important...