Other mistake: When the land is getting destroyed by the volcano, George is in his time machine and the disc attached to it is already spinning before he even turns on the machine and throws the lever forward.
Other mistake: When George climbs into the Time Machine for its test run, he becomes fascinated with the changing fashions of a mannequin in a store window across the street. As he watches the mannequin, the days and nights are flashing by, as indicated by the studio set lights changing from white light to blue light, and by the exterior lights of the building, which switch on at night and off in the day. At first, the shopkeeper appears in the window, also, changing each costume on the mannequin one-by-one. As the shopkeeper labors to change a single costume, the exterior day/night lighting changes three or four times, indicating that the shopkeeper improbably spent three or four days, working day and night, to change just one costume.