Plot hole: When George stops the machine for the first time, the candle burned down to about half of its size, which took according to his observations 98 minutes. But as the trip continues it takes seven hours for it to burn down completely. (00:25:45)
Plot hole: In 1966 when George is about to return to his machine and the atomic bombs (or whatever) go off, George just barely gets out of there before the lava from a nearby volcano cooks him alive. Two plot holes here. One is that in the short time it takes George to walk a few meters to his machine, a volcano explodes and the lava reaches him. Thats very speedy lava..especially considering there were no mountains near George's house. Secondly when the lava approaches, it is just flowing along the ground like normal. But when it reaches George, it splashes around him like a broken water tank. That lava can certainly defy gravity.
Plot hole: George is standing in the small park across from Filby's department store when an atomic weapon detonates. The cars in the street are instantly turned into burned hulks and the building crumbles and bursts into flame. However, George, standing no more than 20 feet away from both, doesn't even break a sweat! While shock waves from a blast can cancel each other out and leave things unhurt, the sheer thermal energy released should have severely burned him at the least.