30 seconds clearly has to be an embellishment, but what tends to happen is that people make a decision pretty early on, and then conduct the rest of the interview in such a way as to make that decision into the correct one. The same answer can be viewed as "he got it mostly right, and in arriving at his answer, he clearly picked out the main salient issue" or "he got the obvious insight about the problem, but fumbled on arriving at a correct solution after that". Which one you go with depends on whether you've already decided the person is generally good or generally poor.

It's usually not a conscious thing, and it's something we're all prone to doing.

Absolutely true, and that's why it's important to make a conscious effort to fight these types of unconscious biases.

That + CV gives an extremely good indication.