An oscilloscope, or "scope" - a fancy voltmeter - being used to test an amplifier circuit that allegedly wasn't working. No problems with it... when it's correctly wired.

This scope is ancient, and the least sophisticated one we have. As it's also the only one of this type we have, I use it for testing stuff in the workshop, and have done so for over a couple of decades. Not the most accurate, but good enough to give an indication of what's happening in many circuits. It's been made good use of, and helped trace many a fault in all kinds of electronic equipment.