Eric's joke reminded me of this one, via Kris McNall:
An engineer, a mechanic, and a computer scientist are driving in a car. Suddenly, the brakes stop working. The car starts rolling down the hill at a terrifying rate, and all 3 scream for their lives. But they get lucky: the car gets to the bottom of the hill safely, and eventually rolls to a stop.
All 3 get out, and the mechanic looks under the hood for about 5 seconds.
The Mechanic: "The problem is obvious! We're missing a Johnson rod. Each of you give me $500, and I'll walk to the nearest garage and get a replacement."
The Engineer: "What? How can you possibly know that? We must completely take apart the car and stress test each individual component to determine the exact point of failure."
Well, the engineer and the mechanic argue for a bit. And then they notice that the computer scientist hasn't been participating in the debate. He's been pushing on the car's front bumper. They ask him why.
The Computer Scientist: "I'm going to push the car back up the hill, get inside, and see if it does it again!"