Hello,
It happens when you program you take the potion when you have already a task in progress : there is a desynchronization between the Client and the server. You can do tasks (not sleep), or moving (short trips of 5 seconds) until it disappear, or taking an elixir.
Sometimes clear Silverlight mermoy is enough (if you play by browser, also clear cache).
You can also write to
support@virtys.com, but your "ghost potion" will probably disappear before someone can do something (I haven't tool for that specific problem). I tried to "resynch" your account but I'm not sure it works.