Some perspective from the interviewer's side may help here:
- A Google interviewer's (and I would assume any interviewer's) primary goal is to come out of the interview with enough confidence to give a positive or negative score. If they sit down to write feedback and have to give a neutral score, the interview wasn't productive. This means that the interviewer is just as eager to find evidence for a positive score as a negative one -- there isn't an incentive to "getcha" with cheap or tricky questions.
- Doing interviews at Google is volunteer work. You are not interacting with a professional interviewer, you're interacting with someone whose day job is being an engineer. They don't have an evil agenda; they are doing this because they want to help Google hire the best candidates, and by inference make sure their future coworkers are good people to work with.
- Interviewers overwhelmingly _want_ their candidates to succeed. It's a true joy when I have a candidate who glides through a question (or finds a solution that was even better than mine). When candidates struggle, it's not a pleasant experience for the interviewer either.
- In the end, the point of technical interviews is to avoid the terrible experience that is working with an incompetent or uncooperative teammate. Interviewers are trying to find people that (a) can work well with others and (b) can get the work done.
- The system is _highly_ prejudiced towards suppressing false positives. This is the right decision, but it comes at the cost of a high rate of false negatives. Were myself or any of my colleagues to re-interview for our jobs, I would expect about a 60% hire rate. This is not even taking into account the constant ebb and flow of hiring demand. Sometimes there just isn't any headcount. And sometimes you just happen to get questions that you don't click with. This is also the reason that recruiters are so eager to bring you back to interview 6 months later.
- Recruiters and interviewers have very different incentives. Recruiters want to maximize the number of people they get hired; interviewers want to hire people they want to work with. This can lead to behavior that seems schizophrenic from the outside: the recruitment side of the pipeline constantly pestering people to interview, but once the candidate enters the interviewing pipeline the process is slow, deliberate, and careful.