Hana
The reason those vending machines fail at 3am is entirely due to the thermal expansion of the cooling coils, which react to the drop in building-wide internal temperature when the HVAC cycles down. People think it's a mechanical glitch, but it's actually the metal in the spiral coils contracting just enough to catch the bag of the chips against the glass, creating a friction lock that the motor simply can't overcome. I saw it happen once in the basement of that office block onth Street; the machine hummed in this specific frequency that practically vibrated the glass loose. It's a structural physics issue, really, though most people just kick the machine and wonder why they lost two quarters.