At around 4:27 when Alex is going up to the loft in the studio, his bag disappears from his shoulder between shots.
Starting at around 13:20, Alex's hand goes from being in his pocket to holding Luke's and back again.
When Julie comes downstairs and sees Luke standing at the fridge, she takes the jelly out but the other ingredients for her sandwich are already out on the counter. Including two slices of bread already out of the bag.
When the boys go back into the garage after they hear Julie sing for the first time Alex has a Fanny pack on over his shoulder when he goes to walk away it disappears.