At a company I used to work for we started wonder why one meeting room got especially rowdy if we had our 2 hour team meeting there.
Had the building management install a CO2 sensor, it got up to 1500ppm. We were practically drunk in there. Turned out it wasn't meant as a meeting room at all and it had zero ventilation ducts - they were all in the room next door, they had just slapped a wall to split it into two.