I accidentally replaced an outlet and added a switch to a circuit that was still energized. I had turned off the wrong breaker, and failed to confirm it before I started work.

But, careful work habits and some tools that happened to be insulated anyway, meant that I was never bridging two different potentials. The job went flawlessly and I only noticed when I plugged the outlet tester into it at the end, expecting to go turn the breaker on and come back and look at the lights... but the lights were already lit up.