The Green P parking garage across the street from the Pleasant Blvd subway entrance was pumping out invalid parking tickets Tuesday afternoon. It caused a nice handful of drivers who couldn’t get out even after they had paid. “It does that sometimes” said a City worker.”We’re not sure why.” His arrival among the trapped motorists was hardly less welcome than the second coming. His advice? Push the help button that lurks among the many messages on the terminal at the exit gates (who knew?) Even so, the presence of invalid tickets seemed a mystery to the person at the other end of the intercom. Green P: “What does it say now?” Driver: “Invalid ticket” Green P: “Try again. What does it say now?” “Driver: “Invalid ticket” The gates were opened most quickly by a polite “Please let me go home.”