Revealing mistake: When David delivers the final kick to Sanga, he raises his left leg to kick but in the next frame it is his right leg which comes out and makes contact to Sanga's face. The right-leg is also a prop made of rubber as you can see the crews hand holding onto the thigh part and the rubber foot flopping/wobbling when contact is made. The following frame shows David bringing back his left leg after the right-legged kick.
Revealing mistake: When Martin is at the rest home, he gets a drink from the water fountain and it sprays him in the face. You can plainly see the spray isn't coming from the fountain itself.
Revealing mistake: On the morning of Bentley's hanging, his father walks into the street and meets the postman. The camera then pulls back to show a streetscape. The gulleys are full of Autumn leaves whereas Bentley was hanged on January 28th. The leaves are there to conceal double yellow lines (which were not introduced in England until February 1956) and in several places they can be seen showing through.