I managed movie theaters for most of the '90's, and was in charge of the concession stand & its staff. One year I came back from vacation to find the employees cleaning the concession stand after a big rush of customers.

I was happy to see this until I realized that some of the staff were using a mop with bleach-based cleanser to clean the floor at the same time other staffers were using an ammonia-based cleanser to clean the glass popcorn bins right next to the employee mopping.

I freaked out a bit, ordering them to stop immediately and turning on a fan to blow the fumes in opposite directions. Apparently neither the employees nor the other manager who had been covering my department while I was on vacation had ever learned that mixing bleach and ammonia can be fatal.

When I asked about it, the other manager apologized but explained that they were simply trying to follow both state and local health/safety board rules. You see, some of the staff were college students while others were minor high school students.