Firing someone over making a mistake is never a good idea. If you're going to have firing offenses, spell those out. E.g. breaking the law, violating some set

Firing someone over making a mistake is never a good idea. If you're going to have firing offenses, spell those out. E.g. breaking the law, violating some set of rules in the handbook, whatever., so that people can at least know there's a process or sensibility to the actions. If people can be fired for making a mistake, and that wasn't laid out at the outset, then they're just not gonna trust the stability of your workplace.