Right after Mrs. Wilkinson slaps Billy on the face, at the locker room, we see her from behind with her hands on her face (at 00:47:33 with fingers as if praying). In the next shot, her hands are in a totally different position (at 00:47:34 with clasped hands on her face).
When the dance teacher goes into Billy's house to confront his family about the audition at the Royal Ballet School, you can hear 'Tellytubbies', a daytime children's programme on the TV in the background. 'Tellytubbies' was first aired in 1997, some 13 years after when the film was set.
In the mobile library scene, several book spines are clearly visible, revealing titles published well after the 1984 setting of the film:
Lisa Appignanesi wrote 'Memory and Desire' in 1991, Barbara Taylor Bradford wrote 'Hold the Dream' in 1985, and 'Shirlee Busbee' wrote 'This Spanish Rose', 1986.
When Billy is confronted by his father who has just found out about his ballet lessons, Billy is wearing a beige shirt under a dark jacket. In the following shots while he runs out onto the street and vents his anger, he is wearing a dark-blue sweater.
The punch-bag in the dimly lit shot of the gymnasium when Mrs Wilkinson is talking to Billy is moving in one shot (at around 39 mins), then stationary in the next (00:39:21), then moving again (00:40:20).
When Jackie is on the bus taking him back to the mines, the strikers pull the protective cage off the window of the right side of the bus. Moments later when the bus pulls into the coal yard, all the windows on the right of the bus have the protective caging.
Billy's brother listens to music in his room (at around 42 mins) while wearing headphones but in cuts to the father and grandmother in other rooms in the house they hear the music as well. Some stereos allow the use of headphones and speakers together.