So is it because they're just careless, because they're working with an abstraction model they're unfamiliar with, because they don't know the necessary math, because they never really grasped formal logic and causality, because they've been given 1 hour to do a 5 hour job, etc, etc. These would not all give the same way to reduce future problems (fire them, wait a couple weeks or give them some training, fire the manager, move their desk from the entrance hallway to a distraction-free office, ...).