The existence and working of Google Sandbox has been debated for long. A WebmasterWorld thread has discussions at length, on the "Minus Thirty Penalty" replacing Google Sandbox. Several people are saying their sites are always ranked at number 31 on Google and perhaps this is some type of penalty placed by Google.
Admin Tedster said: "I've seen urls get a -30 and then seen that penalty removed in stages over several weeks after some condition was fixed. It sure looked automated to me. No, I can't prove that, except to say that Matt Cutts talked about wanting to do some automated penalties with automated removal quite a while ago. And in general, Google always looks to automate wherever they can because "it scales".
Another post suggests : "I just looked at a site that someone on another forum posted the url for, which was hit by the minus thirty penalty. Turns out, that site would be what I would classify as a "thin affiliate". Does that ring any bells with any of your sites?"