A tank vehicle driver approaching a stop sign releases the brake pedal just before the truck comes to a complete stop, and the truck lurches forward slightly. What mistake did the driver make?

49 CFR 383.119(b) (proper braking procedures for the motor vehicle when empty, full, and partially full); AAMVA CDL Manual §8.3.2, p.8-8-2

This is exactly the scenario the manual is warning about: letting go of the brake too early feels natural but it's the moment the surging liquid has the least resistance against it. That small lurch at low speed is a preview of a much bigger, more dangerous push at highway speed.