As soon as the boat carrying Guinevere touches the shore, the rope slackens, even though the other end of the rope is a long way away, attached to a team of horses whipped into a frenzy.
After Guinevere jumps on Lancelot's horse, her skirt changes.
Malagant still has his sword after Lancelot knocks it into the air.
Guinevere's braid (plait) changes after she mounts the white mare for the first time.
Guinevere's dress strap changes after Malagant rips it.
When Malagant shows Guinevere the oubliette he takes a lit torch and drops it into the pit. As the torch gets farther away it begins to fade to black like an ordinary object would. A light source like fire, however, would've stayed brightly lit the entire time.
When Prince Malagant lays his sword on the round table, he doesn't pick it up when he leaves. That was intentional, a sign of his resignation.
Footage of the final fight is obviously sped up.
After Lancelot saves Guinevere the first time, she uses a small crossbow to shoot a guy trying to kiss her. She shoots the crossbow straight up into the guy's stomach. When the guy falls to the ground, the crossbow bolt comes out of the man's stomach horizontally, not vertically.
When the citizens of Camelot gather for the trial of Lancelot and Guinevere. The castle guards keeps the gate open which allows for Malagant and his army to go through the open gate, invade the castle and take it over. Had the castle guards kept the gate closed, put it on heavy security and stayed alert, they would've been able to fend the invading soldiers and King Arthur would've been alive. As a result the castle guards carelessness, King Arthur was killed and the citizens riot against Malagants guards at a heavy loss which could've been avoided had the guards kept the gate closed and stayed more alert.
Lancelot gives Lady Guinevere water to drink from rain gathered in leaves under a large tree. Afterwards, you can clearly see the leaves she drank from are a common house plant.
During the castle fight scene towards the end of the movie there is a moment focused on Lancelot moving in slow motion where he is clearly holding a sword that is just a hilt & pommel but with no blade, obviously just a prop. This mistake was originally only visible in early "pan & scan" versions of the DVD, but is out of frame in "widescreen" editions.
As Lancelot chases the horse a telephone pole is visible, with barbed wire around it.
The movie itself generally takes place in a fantasy world. It does nothing to state a date and the Kingdoms of Camelot and Lyonesse are both without any specific time period other than general medieval. Thus, it is exceptionally petty to argue that a Kingdom that never existed cannot have such and such type of crossbows or bridges.
When Arthur brings Lancelot inside to show him the Round Table, what appears to be a Phillips-head screw is visible on the inside of the door they walk through.
When Lancelot is trying to chase down the white horse, in the background you can see a set of telephone poles and the wires leading of it to the next one.
When the horse is caught, a 17th or 18th century stone bridge is in the background.
When Lancelot and Guinevere are under the tree after escaping from Malagant and she is soaked to the skin you can see her microphone wire on her right leg.
Right before Guinevere jumps from her carriage, you can see the guy who is actually driving the carriage behind the seat. You can see his face through the hole or slit behind the seat and you can see the reins disappear into a slit just below the seat.