How do you accidentally sack someone?

You have to specifically choose a person, presumably with an actual reason for sacking them, then reach out to them specifically to inform them of their sacking.

In my country you also require a valid reason for doing so, you can't just randomly sack someone, I imagine in this case it would be being made redundant.

So, you decide your staff are no longer required and if their contract allows you to let them go (or you just pay the big severance or whatever), okay cool.

Then you.... change your mind? What?

That'd be a deeply incompetent employer. Like, buffoon levels. I wouldn't go back to working for someone like that. If nothing else, they've shown how little they care about their staff. Not worth it.