I have interviewed a lot of people. Most of the time I can determine three questions in if someone is not a good fit. This is on a technical interview. I have never done an interview where someone blew, or gave middling answers to the first three questions, and then recovered. Also, I never gave an interview where the person hit the first three questions out of the park and then started flagging, in terms of technical answers any how.
In terms of people who did well on the first three questions but who did not get an offer, it was always due to "cultural" issues, and it was someone else (often the manager) who raised those cultural issues, with me sometimes disagreeing (for instance, my old manager thought someone was too arrogant, whereas I didn't mind).