I don't touch anything electrical without giving everything a going over with a multimeter. Saved me grief twice. Once when someone (a company electrician) had wired the neutral (not the active) to the main isolation switch to a machine I was working on..
After that incident they installed padlocks on the isolation switch so I had to wait hours for someone to turn up to isolate. I'd do my stuff on the machine then wait hours again for the de isolation in order to check my work. PIA! Especially when I needed multiple isolations to find and fix the fault. Lucky I was paid by the hour but talk about frustrating....