Continuity mistake: The two vials are identified as Valerian sleeping potion (clear liquid) and Henbane (yellow liquid, deadly poison). Gwen pours the yellow liquid in the king's cup, but it's supposed to be the Valerian potion used here that puts the king to sleep. Gwen later dumps the sleeping potion into his ear when it should have been the poison. Then at the end when the Merchant identifies the 2 vials he identifies the yellow as Valerian and the clear vial as Henbane.
xx:xx:xxJohn Wesley Irvine