A little shocking
Recently, I discovered that our porch light had been wired in to our upstairs lighting circuit, instead of the downstairs circuit that it should be part of.
There's no need to ask me how I find that out. You can probably guess.
One of the most frustrating parts of being a homeowner is discovering that the previous owners cut corners or didn't take the same level of pride in their work as you do. "Out of sight, out of mind", is good enough for them.
Anyway, one of the best mindsets you can adopt is that whoever did the job before you did it badly. Kill the power at the fuse box first and always test wiring with a voltage detector before you touch anything!