Hmm... You know how Google said they've been working on the Panda update for about a year? Well, I just came across this:
Amit Singhal: Well, we named it internally after an engineer, and his name is Panda. So internally we called a big Panda. He was one of the key guys. He basically came up with the breakthrough a few months back that made it possible.
The article is dated March 3, 2011. Note that this update's namesake only had the breakthrough "a few months back". Is it possible that a significant factor in this update didn't get quite the rigorous testing and tweaking that we had assumed from Google's "a year in the works" comment?