Every now and again you make a bad hire: even companies with the best hiring processes do. Hence the probation period, and letting this person go during that period.

Not to say we didn't do a retro to see if there were improvements that could be made, but you're never going to be perfect at hiring.

What you do need is robust processes, and the appropriate culture to support them, to deal with the situation quickly on the rare occasions when you do hire someone you shouldn't have.

(Btw, bad hires are extremely costly, even at entry level, not just in terms of hiring costs or productivity, but in terms of the effect they have on people around them: they can be a serious morale drain and, whilst their departure often leads to a bump in morale, the effects can be quite long-lasting. A bad hire in a leadership role is positively dangerous, because you multiply all of the above - and actually it becomes likely you'll lose people who work for them - and on top of this you risk serious erosion of trust in those more senior leaders who hired the person in the first place, which leads to further problems with retention.)