A thought occured to me that the blowout preventers that failed in the BP Gulf of Mexico oil disaster might have been insufficiently robust for an unexpected overpressured sand horizon. Since overpressured zones in the Gulf are not that uncommon, it would be strange if such an occurance were not anticipated. One would expect that maybe the number of overpressured horizons would increase as one moves further offshore.
Granted that many overpressured zones are shales with little porosity and permeability and thus can be handled with little extra difficulty. But at that depth (about 5000 feet water) it seems that the extra precautions wold have been worth it ( if it was indeed the failure of BP to anticipate an overpressured sand).
Just a thought and no way to prove it.