Continuity mistake: In the first exterior shot looking through the Dursley's patio doors, as they have dinner, and later when Vernon runs after Marge, the threshold is wide, flat and all white. Below the glass on the doors the trim is narrow and at the very bottom it is wide. There is a keyhole opening (on the left door) under a particular style door handle and there is a wide double lip going round the entire edge of the left door. When Petunia runs out, the threshold is higher up and mostly brick, with a thin strip of white in the middle. The style of the patio doors under the glass is completely different, there is no keyhole, the handle style differs and there is no double lip on the door. Additionally, the large clay pots with plants, beside the doors, change from being glazed and shiney to being unglazed, its plants differ and things change position.
Picture added by Super Grover