Audio problem: Right after the girls blow up the tanker, there is a sequence with scanner traffic. They show some helicopters. Right at the beginning of the scene with the choppers, the audio of them cuts out for a split second.
Revealing mistake: When they blow up the tanker and drive in circles around the truck driver, the ground has been disturbed from previous takes.
Audio problem: When Thelma approaches the Highway Patrolman's window you can hear her cocking the revolver she then points at his head, but the revolver remains visibly uncocked.
Chosen answer: It's just as you said; he was telling the others to hold their fire. He had no idea that they were going to drive off the cliff; his reaction was evidence of this.
zendaddy621