Tell me about the hardest bug you've shipped a fix for. How did you find the root cause?

Imagine you're trying to find why a toy car stops moving. You check the batteries, the wheels, the motor. The hardest bug is like a hidden switch that only flips when the car goes over a certain rug. You have to methodically test each part until you find that rug. This question shows if you can systematically solve tricky problems.