Continuity mistake: In a wide shot of the corridor when Bugs is running away from the red monster, you can see there is nothing on the wall by the door. When Bugs runs into the door, two wooden beams have appeared.
Continuity mistake: When Bugs strikes the wall with a mallet, the wall crumbles and the red monster falls to the ground. At this point, you can see the mallet is by Bugs' feet, but when we see him in the next shot, the mallet is nowhere to be seen.
Continuity mistake: When the red monster (dressed in knight's armour) gets hit by Bugs' locomotive-style knight-riding horse, he crashes into a wall to turn into a can labelled "Canned Monster." At this point, you can see the can is in front of a plain brick wall, but when we see Bugs looking at the can in the next shot, a purple curtain suddenly appears.
Continuity mistake: When Bugs opens the door to escape from the red monster, the handle on the other side has nothing around it, but in the next shot has a lock has suddenly appeared.
Continuity mistake: The handle that Bugs uses to try to escape was originally not on the door which the scientist locked up when Bugs entered the castle.
Continuity mistake: When the scientist bolts up the door, you can see there is a bull's head near the door, but when Bugs rushes up to the door in order to escape, the bull's head has disappeared.
Continuity mistake: When Bugs kisses the mechanical rabbit, the rabbit short-circuits and breaks into pieces onto the floor. Bugs then starts to leave, but when the scientist pushes Bugs back to meet the monster, the pieces have disappeared.
Continuity mistake: When the scientist locks the door to prevent Bugs from escaping, you can see the latches are made of metal, but in the next shot they are now made of wood.