BP response
In criminal law, some jurisdictions provide the defense of mistake of law. It is a carefully hedged defense, for obvious reasons (it's easy for someone to say, "I got this cryptic information that suggested to me that my actions were perfectly legal"). In Texas, for instance, the defense exists, but to use it, you must have relied on advice given by a government official charged with interpreting the law (a court, for instance, or an attorney general's opinion), and the advice has to be in writing.
I recall only one case that involved this mistake of law concept (and none with mistake of fact): I prosecuted a pollution case against Brown & Root, and its defense was a phone call to the EPA, which allegedly said, "Go ahead and burn all that waste rubber and you won't violate any environmental laws." The judge fined B & R $ 50: a major victory. The criminal defense doesn't merit the heavy treatment that is given to it in casebooks; reading the statute and applying it to a couple of hypothetical problems is actually better for both discussion and retention.
In civil cases, again it depends on the jurisdiction, but the defense is contributory negligence by a third person. Jurisdictions vary about whether a person who cannot be negligent (a child, or the government) can be a contributor, but in Texas, yes. Then, the question is how the causation is apportioned. We have six or seven different apportionment schemes, including one that involves equal proportions to each tortfeasor without regard to percentage causality, but in most cases today, it's probably proportional responsibility (of which there are multiple varieties). So if a jury were to find BP or a contractor 45% causally negligent and the government 45% causally negligent and someone else 10% negligent, liability would be apportioned according to these percentages (probably).
But it's much more complicated, and there are various caps (e.g., a person with less than a certain percentage isn't jointly and severally liable, just liable for that person's proportion).
