Exactly, you have to make sure you ask the correct questions.

For instance, before the iPhone was revealed if you had asked prospective smartphone customers if a hardware keyboard was a mandatory requirement you'd probably get close to 100% agreement. However most customers do not think through the whole design enough to realise what sort of trade-off that would introduce (e.g. smaller screen to accommodate keyboard or less structural integrity due to the need for a slider mechanism etc).

Once the iPhone has been used for a period of time by a user however, I'd guess that less than 5% of users would say a hardware keyboard is a mandatory requirement.

Therefore in such a case you probably asked the wrong question and need to realise when you shouldn't listen to customers answers.