Matt Cutts responded on Google+ that this was not the over optimization penalty, it was a bug with their parked domain classifier. Matt said:
The short explanation is that it turns out that our classifier for parked domains was reading from a couple files which mistakenly were empty. As a result, we classified some sites as parked when they weren't.
I apologize for this; it looks like the issue is fixed now, and we'll look into how to prevent this from happening again.
Some people do not believe it was just a bug with a classifier. Some people think Google was indeed testing the over optimization penalty. Although, they are not doubting that this bug happened around the same time.
It doesn't really matter. Many of the people who complained are not back in the search results. We are still waiting for the over optimization penalty to officially hit. When it does, I am sure many SEOs and webmasters will not be happy. But I am also sure many others will be happy with their new higher rankings.
The bigger issue is that this classifier bug is not new. It happened back in June 2010 and likely happened more times before and after. This time, it seemed to affect a higher percentage of sites, whereas before it affected a smaller number of sites. Matt did say, he will do what he can to make sure it doesn't happen again.
So those 24 hours of lost rankings, lost revenue and lost traffic - well, it was a bug. Google is sorry. At least they admitted it.