I'm still confused by how an AI could run rampant that couldn't be shut off (power, networking, etc) by humans. Like what's an actually realistic scenario for

I'm still confused by how an AI could run rampant that couldn't be shut off (power, networking, etc) by humans. Like what's an actually realistic scenario for that? I guess it could be possible, but I feel like many obvious mistakes would have to be made over time in order to have a machine running that can sustain itself and can't be shut off by humans. At least is seems like that level of autonomy is really far away.