What happens if a driver crosses the service and emergency air-lines when coupling a trailer that has spring brakes?

AAMVA CDL Manual, Section 6.2.4, p. 6-6

This is the real-world consequence of a coupling mistake many drivers wouldn't otherwise connect to a brake problem. If the spring brakes won't release after you push in the air supply knob, the manual tells you exactly where to look, the air-line hookups, instead of assuming the trailer's brake hardware itself has failed.