When Sir Galahad (Michael Palin) arrives at Castle Anthrax, he is told by Zoot that "8 score" (160) blondes and brunettes reside there. Later on, when Sir Lancelot comes to "rescue" him, he protests that he can handle "150" girls.
During the sword fight between King Arthur and the Black Knight, the lighting repeatedly changes between overcast and a bright sunny day, and the fire in the tent in the background changes from lit to unlit to lit.
After Sir Lancelot's wedding-crashing, the bride is first seen with no blood on her face. Then a close-up shows her with bright red blood coming from her mouth and running onto her chin. In the next shot, the blood is gone again.
After being attacked by the white rabbit, the knights drop their shields and run away. Sir Galahad drops his in front of the cave opening, but in the next shot, it's back on his arm. Afterwards they go back to the entrance and the knights' shields are nowhere to be seen.
During the second attack of the killer rabbit, the rabbit attacks the neck of the knight who is wearing Sir Lancelot's armor. At the end of the battle, Sir Lancelot has made it out with the rest of the knights and is still alive.
When King Arthur is watching the Black Knight fight, the Black Knight throws a sword straight into the visor of the other knight's helmet. Later, when he takes his sword out, the sword is sideways in the visor.
During the "AAAAAAAAAGGHH" cave scene, the actor who substitutes Eric Idle as Sir Robin (as Idle is busy speaking as Brother Maynard), suddenly vanishes when the camera change position to the back of the actors.
After Lancelot rescues Galahad from Castle Anthrax, a crew member is briefly visible shortly after the knights go around an outside corner of the castle, just before the scene ends (look on the left side of the screen). Since breaking the fourth wall does not serve a humorous purpose here, it is a goof.
Near the end of the Black Knight scene, King Arthur says to the Black Knight, "We must cross that bridge." Just behind the Black Knight for a brief moment is seen a small stream, no more than 2 feet across which could easily be stepped over. No bridge is in sight and wouldn't be needed for such a tiny stream.
The Knights Who Say Ni claim to be afraid of the word "it," yet everyone present has already said "it" several times, with no reaction. In Monty Python gags, it is common that characters with peculiar oddities are only afflicted when it's most convenient for the sketch.
Towards the end of Brother Maynard reading the message in the cave, Sir Robin lowers his shield, and it can be seen that he is played by a stand-in and not Eric Idle, who is playing Brother Maynard instead.