Will pulls and releases the slide on his semi-automatic pistol twice (once in the lavatory and once outside the uncle's flat), after he's already fired it in earlier scenes. This would have ejected a round already in the chamber.
Will and his dad Martin drive to an apartment to get a gun, they park the car that looks like a Seat MPV or something with 7075 GXF on the license plate. They get out and drive the same car away, but when they arrive to meet Jean Carrack, the car is an Audi SUV with 1892 GXA on the plate.
When Will and Lucia are travelling by train to Puerta del Sol, Lucia is wearing a blue jacket. When she says goodbye to Will in the train station, the jacket is gone. Lucia is neither wearing nor holding it, nor does she have it tied around her waist.