The one I remember most is, when experimenting with Opus 3.5 for the first time, I asked it to generate a Firecracker backed local VM creation and management tool, something I'd wanted for a while but not found.

My expectation was that it might get something barely functional but would probably fail, and instead it generated a working piece of software which achieved a lot of what I wanted.

That definitely made me realise that, for at least some classes of software task this was a major change in how things could be done.

More recently when I can give the model a Local Privilege Escalation PoC in Linux and ask it to test whether it can be used for container breakout and then generate a working container breakout, all in one prompt... that definitely changes things.