When Red drops the broken half of the spark plug into the mower's gas tank, it splashes a little of the gas out, indicating that the tank is full. Yet a moment later when Red checks to see if there is any gas left in the tank by holding a lighted match near the tank's opening, there is no resulting geyser of flame, and Red states that the tank is empty.
When Jack first comes up out of his cave, he's in full sun. The next shot is from a different angel and there's no sun on the rocks or ground around him. All the subsequent shots are back in full sun.