The Breaker Finder 5000
I would claim "I could write a book" but I did HVAC service and installs, and was never a licensed electrician. I'm sure some of THOSE guys could REALLY tell stories. Even so, I've sure seen my share.
But I've done a few (temporary) shaky things myself.
Probably the worst was way back when I was temporarily rooming with a gal, and needed to do a quick weld. The range dryer was way around back, I did not have enough wire. I had some no 10 Romex that I had previously used at my old place for welder extension. Her panel was off near the front and side of the house, down an enclosed outside stairs into the basement. She was gone shopping, nobody around.
So I just pulled the main, vise grip clamped the no 10 direct to the main busses, snapped the main back on, and made the weld. So you've got 100-200A main buss bars feeding a no10 wire.
One other time I was testing an amateur radio RF amplifier, wired for 220. I had no 220 on that side of the house, but I knew that two nearby outlets were on different breakers, and so I checked them, hot terminal of one to hot of the other, and sure enough, they were on opposite phases, so 220. I just wired the amp up to the two outlets and away we go.
Nobody but me, no kids, no dogs.
NEVER assume a cold hot is cold, that a neutral is cold, or that a ground is ground.