I can remember when we were setting up a mic for a school show. No matter what the tester did, it didn't seem to pick anything up. We were standing in front of it arguing what was wrong, when the sound guy stuck his head out and told us he could hear our argument perfectly. It was a directional mic. The problem was solved by turning it round.
This one cracked me up. I have countless stories of several times in which I have struggled to fix really small issues but had to take hours to find a solution.