Working in production is a hell of a thing. You're disarming a bomb; red wire? green wire? blue wire? It's especially nerve-wracking if you cannot reproduce the issue in any other environment and the customer wants a resolution stat. You know it's not the green wire or the blue wire, because you wired those yourself and know exactly where they run. Compound the pressure with the muddy waters of knowledge when there are multiple people on a project. Someone else ran the red wire, but you've traced it prior and are fairly certain it's not the right one. Then, your light bounces off something you'd not noticed before: someone else did something in production that changed the bomb. Now there's a yellow wire and a black wire.
Throw on your cowboy hat. Cut.