In the 1962 Harmony town street scenes, the same dozen or so cars and trucks are seen driving around as if they are simply circling around or turning around and going in the opposite direction. They are in differing orders and groupings but are always the same vehicles.
The tiny "barber pole" on the front of the time machine helmet vanishes and then reappears.
In the repair shop, when supposedly setting the dial on the time machine helmet to go back to 1890, one could see it was still set for 1962.
There was a rotary telephone on the counter at the repair shop that was off of the hook, yet it did not make any noise (like a dial tone). The phone later returns to the hook with nobody around.
When Buster Keaton's character sends Rollo back to 1962, he should stay there only 30 minutes before bouncing back to 1890.