In the scene where the boy is making breakfast for his mum, he makes her a bowl of cereal, some toast, and a glass of orange juice. Keep your eye on the orange juice because after the boy opens the curtains, the orange juice turns into milk. This slip-up is at seven minutes into the movie and goes for about two minutes and 15 seconds.
I just went to take pictures of this slip-up, and after watching the scene twice in a row, I can tell you that the orange juice never changes to milk. It is not a slip-up at all.