The US FAA announced about a year ago that a pilot is responsible for the clearance as given by the controller. If a pilot reads the clearance back wrong, the controller has no obligation to correct him and the pilot is still legally responsible to follow what the controller issued, even though he doesn't know he misunderstood it. Great system, huh?
So much for checks and balances. The NTSB is against this FAA ruling, but since they are not a regulatory organization, the FAA can ignore them.