A mistake is made, and a material loss has been incurred. This sucks. Been there, done that, didn't get the t-shirt because we couldn't afford such a luxury. I watched my annual bonus evaporate because of somebody else's cock-up.

But there's no reason to believe that firing the mistake-maker is the best move. Maybe the right move is to find money somewhere else (cutting a bunch of discretionary, pushing some expenses into the future, reducing some investment), or maybe it's to ask a few people to take pay cuts in return for some deferred comp. Or maybe it's to lay-off somebody who didn't do anything wrong, but who provides less marginal value to the company.

But it'd be one hell of a coincidence if, after an honest to god mistake, the best next action was to fire the person who made the mistake. After all, if they were in a position to screw your company that hard, they almost certainly had a history of being talented, highly valued, and trustworthy. If they weren't good, you wouldn't have put them in a place where failure is so devastating.