I was replacing a bunch of outlets in one of my apartment units last month. Everything was going fine, too fine. It got to a point where I got complacent, a factor in MANY deaths.
I shut off all the sub-circuits for the downstairs and went to each outlet, testing for power, then replacing it. As I'm doing this, I eventually hit that complacency point and stop testing them. Like a pilot who skips a "needless" step in their preflight, doesn't die, then makes skipping that step a part of the usual process, I changed a few outlets, wasn't shocked, and thought nothing more about it.
...until I got to the lightswitch that controlled the disposal in the kitchen. Krr-ZAP! Numb hand, twitchy muscles, and a very surprised Chairboy. Turns out it shared a circuit with the upstairs.
I learned the obvious lessons, plus connected it with my "dangerous personality factors" list that I apply to flying, driving, etc. A good cheap lesson is the kind that hurt enough for you to notice, but doesn't kill you or put you in the hospital.