(at around 7 mins) After Arthur takes a deep smell of the cloth and awakens he asks "what is that!". Merlin tells him "a combination of hogwort and valerian". Hogwort contains Croton oil which is caustic, causing irritation and pain even to the point of blisters in high enough concentrations and would keep someone awake due to the resulting discomfort if inhaled. St John's Wort however is used to induce sleep, as is valerian, and could in high enough concentrations (and with a bit of magick) be used to knock someone out in order to kidnap them; something that any warlock like Merlin would know.
Cendren takes out his dagger twice as he heads towards Gwen to remove her restraints .
When Arthur is trying to light a fire in the forest he uses a modern Ferrocerium rod (aka "ferro rod" or firestick) which is a 20th century invention.
When Gwen leaves Morgana's chambers, she is wearing a modern band-aid/plaster on her upper right arm, visible through the sheer fabric of her sleeve.