Steve is released from jail "in the middle of the night", but when he is subsequently fighting with the sheriff and the gunman, in the background it is bright daylight with a blue sky.
When Steve knocks out the tanker's first gas-filled false bottom, yellowish fluid splashes out, correctly indicating the pocket of gasoline that he's broken. But then when Steve punches out the second false bottom, only clear fluid splashes out, indicating that the cavity is filled with water or hooch, not gas. In addition, Steve splashed gas all over himself in the process, which would be a dead giveaway afterwards when the Sheriff checked on him in jail.
Steve has bent the two bars of his jail cell away from each other in a broad curve, yet later in a close-up shot of him finishing straightening the right-hand bar, the left-hand bar is already straight, even though he hasn't straightened it yet. Then in the next shot, the left-hand bar is bent again.
Steve bends the car's roll-bar to pin the two mercs back against their seats, yet there is a good space to the sides of the bends where they could just tilt their heads and get free of the bent bar. But then in a later shot of Steve showing them the pebbles that he'd used to simulate a snake's rattle, the bar is bent around their necks a lot more closely.
If the big tanker truck was mostly full of illegal moonshine, then just two or three gallons of gas in the tanks would not be immediately and dramatically detectable; at most, the hooch would merely taste a trifle "off", not almost like pure gas.