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.
Continuity mistake: When George stops his time machine at 802701 A.D. he stops it too fast and falls out of his time machine. While he's lying on the ground rain falls out of the sky and only lands on him and a little bit of the ground. When George stands up however his hair and his clothes are completely dry and steam is rising from the ground.
Factual error: When George starts time traveling in his lab, we see the sun and moon cross his field of view. But then we see stars circling Polaris in the same area. Polaris is nowhere near the path of the sun.
Continuity mistake: When George stops in 1917 he kicks some planks from a fence that has 4 small signs and a plank missing, in the next shot 3 signs are gone and the missing plank is reattached. After he talks with Filby Jr. he climbs back through the original fence (the one with the missing plank and 4 small signs, plus 3 other ones that were out of sight in the first shot). (00:34:00)
Continuity mistake: When the Time Traveller gets back into the machine after the volcano scene, because the numbers are turning so fast, you can only see the first number; the first time you see this, the first number says 8, then 9, indicating 8,000 then 9,000 years, then it cuts to the Traveller for a second, and when it is seen again, it says 8,000 then 9,000 then 10,000.
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.