Reg Kray takes Frances into her parents' garden where a 'decorative' garden gnome can be seen between them. Garden gnomes of the early 1960s were cast in plaster and painted but this one is of moulded plastic, a method of manufacture that wasn't used for this purpose until later.
During the school playground fight scene between the Krays and some other kids a crew member wearing a black leather jacket and white trousers can be seen running out of shot in the top right hand corner of the screen. Watch carefully as he is quite fast.
In the boxing scene at the fairground, the twins' are portrayed as young adults and that the fight between them transpires after Ronnie knocks out the prize fighter - this incident actually took place when they were both small boys. The ringmaster shouted to the crowd if anyone wanted to take on the show fighter and the young Ronnie Kray shouted that he'd fight him. Amid much laughter, the referee pointedly said he was a bit young - whereupon Reggie stepped up and challenged his brother. They fought gamely, and were both awarded half a crown for their efforts; this was also the bout that led to their later semi-professional careers as pugilists.
The movie depicts the murders of George Cornell and Jack "The Hat" McVitie as taking place on the same evening. In reality, George Cornell was shot by Ron Kray on 9th March 1966 and Jack McVitie was stabbed to death by Reg Kray on 29th October 1967.
In the slow-motion sequences of the fairground boxing scene, the filming lights illuminating the scene can be seen flickering very prominently, due to beating between the mains frequency of the lights and the very high frame rate used to achieve the slow motion. They should have used DC-powered lights!
The goof items below may give away important plot points.
Violet's funeral takes place on a foggy winter's day with bare trees visible in the cemetery. But in reality it was in August, as is shown by a close-up of the gravestone. (The date of death is given as 4th August, and the funeral was in fact a week later, on the 11th).
The closing moments of the film show The Kray Twins standing at the graveside during their mother's funeral while a vicar performs the Last Rites. Whilst it is true that The Krays were granted permission to attend their mother's funeral, they were not permitted to attend the graveside service.