"Deepwater Horizon 's containment failure may have come in the cement injected between the well wall and its steel liner, experts say. According to press reports, workers had just finished that "cement job." A failure could have also come in the temporary cement plug set in the well. Problems with cement—a batch that fails to properly set up, for example—were the cause of 18 of 39 blowouts in the Gulf of Mexico over a 14-year period, according to a 2007 report from the U.S. Minerals Management Service, the agency that supervises offshore drilling."
One thing is for sure if the Horizon wasn't operating at it's maximum designed level, I imagine if it had been and this occurred, the shutting off of a catastrophic gusher would be even more difficult than it is now.
Thanks for the thread, tbredbeck.