1. Extra interviews required. This has nothing to do with the HC. In many cases, interviewers themselves screwed up here, and told the HC so. The alternative would have been "no-hire." 2. Candidate got redirected to a different department. This resets the hiring clock. This is unfortunate, but the alternative would have been a straight "no-hire." 3. HC too over-committed. I wrote a bit about this in my book and won't go into it here. It's not an easy problem, and Google's scaling problems came into play here.
Reed Hastings makes a case that as a company grows, hiring standards should go up: http://www.slideshare.net/reed2001/culture-1798664. I agree, and if the consequences is that you have a harder time making a quick decision and have to lose some otherwise good candidates, I think the quick decision is the greater of the two evils.
This is especially true in Google's case, since there's no shortage of candidates willing to wait. Bear in mind that many candidates aren't really waiting: they already have a job and an extra month of latency won't reduce their interests. My brother worked at Intel while applying to Google. It took us 3 months to hire him. While it wasn't fun for him, it also wasn't costly either. (And yes, he took the job)