From a Googler: the Google interview process(reddit.com)

  I used to care much more. I used to try to help them. Try
  to make them feel good. But I can talk and talk, explain 
  and explain, in the end we won’t hire you if you can’t 
  reverse a linked list, or do a case-insensitive string 
  comparison. I have done this so many times, I'm terribly 
  frustrated about this. So now if you fuck up here, I’ll 
  just let you talk for 20 minutes, say “uh huh” once in a 
  while and review code in the meantime. And then I’ll ask 
  you about “your most interesting project so far” or some 
  bullshit like that.

  ...you will have 3 interviews of 45 minutes, lunch, and 2 
  more interviews. These are basically the same as phone 
  screens, but you get to see the interviewers face to face.
  
  If you totally suck, they sometimes walk you out after 
  lunch, and skip the last two interviews.

  ...the interviewer would rather stab himself than hear you 
  fail again. So he lets you tell a story while he zones 
  out. In his rating, he will give you the lowest possible 
  score, and this will end the interview process for you.