Watch the movie near the end after King Richard gets shot with the arrow and the whole swordfight deal with Lancelot and Maligant starts up. Keep watching right to the end of the battle where Lancelot slays Maligant and Maligant falls into the throne. Look at the bottom of the screen when Lancelot comes back on screen put the VCR on slo-mo. Lancelot will raise Excalibur until it appears as though the blade has broken off.

In the making of the movie, Richard Gere wasn't strong enough to lift a real sword so they made him a hilt to work with and computer generated the rest.

If Excalibur was unbreakable, then how come the blade broke off? Y2K Bug? You tell me!