During Johnny's big emotional speech to his former Cobra Kai students, the four girls in the background continue to change positions, leave, then reappear multiple times.
During the fishing scene, Daniel's line is clearly not in the water. At one point the fishing line is actually rising above the pole.
During the scene in the park when the students receive the new t-shirts, several red t-shirts can be seen on the floor in the close up shots but disappear for the wide shots.