Update: the $8 inspection mirror got me out of a job I was ready to write off
Had a Cessna 172 in the hangar off Airport Road last Tuesday with a fuel smell nobody could pin down after two pressure checks. I kept pulling panels and getting nowhere, burning almost 3 hours on my knees. Then I grabbed one of those little telescoping mirrors with the LED on it, the cheap one from the parts trailer, and snaked it behind the wing root. Turned out a vent line clamp was loose in a spot my eyes just could not get to. Fifteen minutes later the leak was done and signed off. I used to think those mirrors were a gimmick for the new kids, but now it lives in my front pocket. What small tool ended up saving you a whole afternoon of guessing?