When Bugs runs into the door at the base of the tower (chased by Sam the Black Knight and the dragon), he is holding the Singing Sword. When he ducks out and locks his adversaries inside, he is missing the sword. Later, leaving the castle, he has it again.
When Yosemite Sam climbs on the catapult, his sword suddenly appears out of nowhere which he uses to cut the rope.
When Bugs says goodbye to Sam and the dragon at the end of the cartoon, his lips never move to any of his various farewells.