Visible crew/equipment: After Arthur drinks from the Grail and goes to visit Gwinevere at the convent and she gives him the sword back, he paces around her chamber- you can clearly see the film crew reflected in his armor.
Audio problem: When Uther takes the baby from Igrayne and gives it to Merlin, Igrayne's screaming and whimpering is not consistent with her mouth movements.
Continuity mistake: As King Arthur addresses the Knights of the Round Table he says, "Only the Grail can restore leaf and flower" and Perceval and the other nights are facing him. In the next shot Perceval and the knight to the left of him are facing forward with only their heads turned towards Arthur. (01:35:30)
Continuity mistake: King Arthur asks Merlin where evil is in his kingdom and he replies, "Always where you never expect it, always". In the next shot Morgana walks back to her seat at the table as Gawain blurts out, "I know where" but then two shots later we see her back at the wall where she was speaking to Merlin. (01:10:20)
Other mistake: When Arthur and all his knights assemble on hilltop at night and Merlin tells them all not to forget where the Round Table started, you can see the crew's spotlights reflected on the back of some of the knights' armour.
Continuity mistake: As Morgana and Merlin speak, the position of the vial that she's wearing changes between shots. In the shots where we see Merlin's face a gold spec (many are included as decorations in the veil) is next to her cheek. In the shots where we see Morgana's face the gold spec moves and is next to her right temple. (01:01:45)
Continuity mistake: King Arthur's second horseman carries three extra jousting sticks as they ride to the far end of the field preparing for the joust. After the first charge Lancelot says, "You joust well, Sir, I offer you another lance" and King Arthur rides back to his second horseman who now only has one jousting stick. (00:48:15)
Continuity mistake: During the knight's feast at the round table Morgana and Merlin go off to chat and he says to her, "I've shown you too much already". We see three dogs (one lying down and two standing) bathed in a green light behind his head. Two seconds later we see this spot again and there are two dogs standing in natural light. (01:09:30)