Continuity

Hair, clothing, props, the level in a glass, which hand something is in, the direction the light falls from. All of it has to match across shots meant to read as one continuous moment, even when they were filmed hours or days apart. Creators run into it hardest on multi-day shoots and on pickups, where you sit down to redo one line a week later and the haircut, the jumper and the daylight have all moved on. A reference photo at the end of every setup solves most of it for free. Small mismatches vanish in a fast cut and become the only thing anyone sees in a slow one.