This is a story for those of us who think, how is this possible?, (the customer) does not know what he/she is doing, something else must be wrong It’s a legendary tale from the archives of General Motors that has become a classic in engineering, customer support, testing and problem-solving circles.

It began with a complaint received by the Pontiac Division of General Motors. The letter writer was polite but insistent:

“This is the second time I have written you, and I don’t blame you for not answering me, because I kind of sounded crazy, but it is a fact that we have a tradition in our family of ice cream for dessert after dinner each night. But the kind of ice cream varies so, every night, after we’ve eaten, the whole family votes on which kind of ice cream we should have and I drive down to the store to get it.

It’s also a fact that I recently purchased a new Pontiac and since then my trips to the store have created a problem. You see, every time I buy vanilla ice cream, when I start back from the store my car won’t start. If I get any other kind of ice cream, the car starts just fine.

I want you to know I’m serious about this question, no matter how silly it sounds: ‘What is there about a Pontiac that makes it not start when I get vanilla ice cream, and easy to start whenever I get any other kind?’”

The letter landed on the desk of the Pontiac President, who was, understandably, skeptical. It sounded like a prank. But, on the off-chance it wasn’t, he sent an engineer to investigate.

The engineer arrived at the man’s home in a fine neighborhood and was greeted by a successful, well-educated man. This was no crank. The timing was arranged for after dinner, and the two of them drove to the ice cream store. The family had voted for vanilla. They bought the ice cream, returned to the car, and, just as the letter described, it wouldn’t start.

The intrigued engineer decided to continue the investigation. He returned for three more nights.

  • Night 1: Chocolate. The car started.
  • Night 2: Strawberry. The car started.
  • Night 3: Vanilla. The car failed to start.

The engineer, a logical man, refused to believe the car was allergic to vanilla ice cream. He knew there had to be a rational explanation. He started taking meticulous notes: time of day, type of gas used, time to drive to and from the store, and so on.

Soon, a clue emerged from his data: the man took significantly less time to buy vanilla ice cream than any other flavor.

Why? The layout of the store was the reason.

Vanilla was the most popular flavor, so the store kept it in a separate freezer case at the very front for quick and easy pickup. All the other flavors were in the back of the store, requiring customers to walk further, wait longer at the counter, and take more time to check out.

The problem wasn’t the flavor of the ice cream; it was the time it took to buy it.

The question now became: why would a shorter stop prevent the car from starting? The engineer quickly deduced the answer: vapor lock.

The car’s engine didn’t have enough time to cool down during the quick trips for vanilla. The engine remained hot, causing the gasoline to vaporize in the fuel line. This bubble of gas—the vapor lock—prevented the flow of fuel to the engine, so it couldn’t start. On nights when other flavors were purchased, the longer time spent in the store allowed the engine to cool down just enough for the vapor lock to dissipate.

The moral of the story is simple: even insane-looking problems are sometimes real.

In software development, we often receive bug reports that seem just as outlandish. It’s easy to dismiss them as user error or a misunderstanding. But this story reminds us to treat every issue as potentially valid. The user might not know why it’s happening, but they are experiencing a real problem. Our job, like that of the Pontiac engineer, is to dig deeper, gather the data, and find the logical cause behind the seemingly illogical symptom.

So, the next time you get a bug report that sounds like a car being allergic to vanilla ice cream, give it your full attention. You might just uncover your own “vapor lock.”