Judging by his tattoos and his necklace, Boyka is clearly an Orthodox Christian. Yet when he is praying in his cell, he does the Sign of the Cross the way Catholic Christians do it, touching his forehead, then his left shoulder and finally his right shoulder. Orthodox Christians touch their right shoulder and then their left shoulder when making the Sign.
In the end scene where Boyka steps out of the car and has a small conversation with Turbo, Boyka is shown standing near the one side of the car. But when Turbo closes the door and the car leaves, Boyka is shown standing and looking at it from behind.
During the last fight scene between Uri Boyka (Adkins) and Raul 'Dolor' Quinones (Zaror) just after Boyka is knocked out of the ring, Dolor turns around with his arms spread to receive praise from the crowd, his back tattoo has been smeared off. When Boyka comes back into the ring with the "bandage" on his leg, we can see that the tattoo has been restored.
During the fight in the rock yard where Turbo is trying to escape, you can see that the baton he is hitting one of the prison guards with is a fake prop as it clearly bends back and forth when he swung it.