Spent 4 hours chasing a hydraulic leak on a 737 main gear door only to find it was a loose O-ring that fell out during install | Aircraft Mechanics - The Community Guide
Man, that hurts... Three days once on a Cessna 172, turned out the ground wire was just loose at the battery terminal. Felt like such a fool when I finally saw it. Another time spent two hours chasing a no-start on a diesel truck, ended up being the kill switch was bumped off. It's always the stupid stuff that eats up your whole shift.