When I tried thinking about this after watching the movie I always thought that somehow I had mixed something up, but no, there is a weird thing that happens halfway through the clip. When they break the chalice from the palace the flagon with the dragon has the pellet with the poison, but the pellet with the poison was in the vessel with the pessel, which now has the brew that is true. So, when they broke the chalice from the palace they also emptied the vessel with the pessel?
Regardless, it's a great routine and something that I've been known to say at work and utterly confuse people.