When Will is shopping and he hears "Santa's Super Sleigh" being played, he says to himself "November the sodding nineteenth. Six weeks before Christmas and already they were playing the bloody thing." In fact, November 19th is exactly five weeks before Christmas Eve, not six.
When Marcus is watching his mother cry in the kitchen while she prepares breakfast towards the start of the film, a centre parting appears in his fringe that was not previously there, then disappears and reappears.
When Will is confronted by the restaurant maitre d' for disturbing the other customers, the maitre d' first holds up his left arm close to his chest, then it switches to his right arm, then back to his left.
The film appears to take place from mid - Autumn through the Christmas season, into the new year. However, the foliage in the background clearly indicates a late Winter through Spring filming, with scenes clearly shot out of sequence