Goofs

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.

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.

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.

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.

At 36:14, as Lancelot catches up to the loose white horse, and again at 36:21 as 'Peter, King's Stableman' approaches, another modern utility pole is in the background, this one with what is likely barbed wire fencing nearby (the posts are visible with nothing solid between them).

Simple wire fencing was first used in the early 1800s, and barbed wire was invented in the 1860s or 70s (depending on where one reads).

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.

At 44:36, when Arthur brings Lancelot inside to talk about the gauntlet before showing him the Round Table, what appears to be a Phillips-head screw is visible on the inside of the stationary (viewers' left) door in the first doorway they walk through, approximately in the center of the visible part of that door.

At 36:06, when Lancelot is on horseback chasing down the white horse running loose, in the background you can see modern utility poles and the wires between them.

In the foreground, there is also some kind of wire installed along the side of the embankment facing where the white horse is running.

Confetti was actually first used in Paris, France in 1891. Not this early in history.

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.