brianp - 11:44 pm on May 25, 2013 (gmt 0)
A few details I did not include:
We saw a HUGE spike in crawling activity the week before this update hit. 30,000+ pages crawled per day, when our website has maybe 2500 total.
Searching Google for some of the mistakenly created URL's definitely shows many more were indexed that weren't listed iN WMT.
If I had to guess, I'd say the 5/7 update was perhaps when the adding Panda to the main algo was implemented. Everything I've found thus far looks more Panda like than anything else.