Let me share one of my experiece where I could not qualify an interview.

It was a technical interview organized on Sunday. I was the first one among the list of the candidates. There were 3 panels taking Interviews parallely. In my luck, there was one of the delivery manager of the account. Eventhough, he was too qualified to interview a candidate like me, I had no options but to face him. Anyway, the kind of questions that were asked from me were too abstract and sometimes too into the deep for my kind of experience. I tried to answer them as per my capacity but could not satisfy him :-(

I attended with the same kind of preparation some other Interviews as well and got through them easily. Need not mention, at those occassions, I was up against some Right kind of interviewer and he was delighted with my responses.

So, the take away from my experience, we should try to judge the situation and act accordingly. Try to understand the scenario, the expectations of the interviewer. Never hurry up in your reply. Once you match your frequency with them, try to take the command. If you realize at any point of time that things being asked are too vague, just don't jump into the conclusion that your fate has been decided. Instead fight by supporting your arguments. In the end, it is you who should be satisfied even though you lose the battle :-)