I've had a tangential interest in the physics / mechanics of air-conditioning since working in an office with two people on opposite ends of the "comfortable temperature" scale, and noticing that one of them opens their window in order to "solve the problem". I've since noticed various other people open windows to try to "fix" the air-conditioning temperature.

My assumption is that opening the window fucks it up for the whole system, but very few people are aware of this, and the office folks aren't made aware of it by the air-con techs.

Am I way off? Am I overstating the effect that an open window has on the system?

It will upset the building balance at a minimum. Air balancing techs are typically a different discipline from HVAC in large commercial spaces.

Leaving the window open and AC on in high humidity areas can freeze up the coil generating a service call, trick zone sensors, etc. If there is a large temperature difference the sharp gradient will not be comfortable.

I never worked on very large systems, mostly small commercial and residential. A buddy of mine supervises HVAC for a large commercial facility and we were talking about their makeup air. Basically it sounded like the building is effectively hermetically sealed until the makeup air vents open because of interior pressure or O2/CO2 levels. Seemed kind of creepy that there was some active system in charge of that. How do you know when it's not working?

The same CO2 sensors that call for make up air are hopefully monitored. Sick building syndrome.

Buildings are poorly sealed in my experience. In California code requires economizers on RTUs and they are almost always broken / stuck partially open. In the Inland Empire for example the actuators bind up from sand and often seize. Nobody wants to pay to fix just that, if they even know it’s busted.

I stay under 10 tons. Things are different in bigger buildings!