It's a story that so easily demonstrates the difference between a mistake of fact and a mistake of law that you might think it was a hypothetical, made up just for that purpose.
In case the facts in Ostrowski v Palmer don't immediately spring to mind, here's a short version of them...
Jeffrey Palmer was a professional rock-lobster fisherman. On 13 November 1998 he went to the Fisheries WA office in Fremantle. He asked for the current regulations to cover the 1998/99 season. They weren't available, but the person he spoke with at the reception counter provided him documents and pamphlets. Unfortunately for Mr Palmer, none of the material mentioned a particular regulation. The effect of that particular regulation was to prohibit commercial rock-lobster fishing at a place called Quobba Point, which (if Google maps is accurate) is a little north of Canarvon, about 900 km north of Perth.
On 7 and 9 February 1999, Mr Palmer placed rock-lobster pots near Quobba Point. On 10 February, Fisheries officers spoke with him, and George Ostrowski later charged him.
The charge was heard on 1 March 2000. The magistrate rejected the defence of honest and reasonable mistake of fact (in WA, found in Criminal Code s 24). He imposed an obligatory fine of $27 600, as well as a further $500 fine, and awarded $2000 costs against him. The magistrate specifically found that Mr Palmer acted honestly and reasonably, but his mistake was one of law, not fact.
Mr Palmer appealed to the Full Court of the Supreme Court of Western Australia.
In a 2:1 decision, the Full Court found that Mr Palmer's mistake was one of fact.
Fisheries WA appealed to the High Court.
The High Court found, 5:0, Jeffrey Palmer's mistake was one of law, not fact.
Gleeson CJ and Kirby J emphasised the importance of identifying the elements of the offence to be proved.
 [I]f a person is alleged to have committed an offence, it is both necessary and sufficient for the prosecution to prove the elements of the offence, and it is irrelevant to the question of guilt that the accused person was not aware that those elements constituted an offence.
They held that the only mistake Mr Palmer made was a mistake resulting from his ignorance of the law, at .
McHugh J came to the same conclusion.
 In response, Mr Palmer contends that the earlier dealings with Fisheries WA are relevant — indeed fundamental — to characterising his mistake. He submits that at all material times he possessed an honest and reasonable, but mistaken, belief that:(a) he had been provided with all relevant regulations concerning the area where he proposed to fish;(b) there was no regulation bearing on the closure by Fisheries WA of the waters in which he proposed to fish or, in short, that reg 34 of the regulations did not exist; and(c) he was lawfully entitled to fish where he did....
 At common law, and in my opinion under the Criminal Code, once the prosecution proves in relation to a strict liability offence that the defendant knew the facts that constitute the actus reus of the offence, that is, all the facts constituting the ingredients necessary to make the act criminal, the defendant cannot escape criminal responsibility by contending that he or she did not understand the legal consequences of those facts. In R v Turnbull, Jordan CJ, when discussing the common law concept of mens rea, said:[I]t is also necessary at common law for the prosecution to prove that he knew that he was doing the criminal act which is charged against him, that is, that he knew that all the facts constituting the ingredients necessary to make the act criminal were involved in what he was doing. If this be established, it is no defence that he did not know that the act which he was consciously doing was forbidden by law. Ignorance of the law is no excuse. But it is a good defence if he displaces the evidence relied upon as establishing his knowledge of the presence of some essential factual ingredient of the crime charged. [Emphasis added]
This passage was cited with approval by Brennan J in He Kaw Teh v R.
Callinan and Heydon J came to the same conclusion.
 The difficulty for the respondent is that there were here a series of mistakes, the one to which Olsson AUJ referred, the actual decision to rely on the information with which he had been provided, and the actual reliance, by fishing in the embargoed waters. The last is a different mistake from, for example, a mistake as to the location of his vessel or his lobster pots. The last, it can be seen, is discrete in time, place and physical activity from the other two, although but for them it is unlikely that it would have been made. The offence of which the respondent was convicted was not of failing to obtain, or hold and rely on complete and accurate materials, but of fishing where professional fishing was impermissible. The elements of the offence consisted of fishing in the embargoed waters, an activity which the respondent knew to be proscribed. Unfortunately, in the circumstances he could be no less guilty than a motorist who has done everything reasonably possible to ascertain the speed limits on a stretch of roadway along which he is to travel but having failed to do so, in one or more instances, exceeds those limits because he was unaware of them.
(They were also pretty scathing of the exercise of prosecutorial discretion to commence the proceedings.)
The case shows how important it is to figure out if a mistake is one of law or fact. And, how hard it can be. In this case, when the dust settled 5½ years after Jeffrey Palmer fished for rock-lobster, 9 judicial officers had considered the case and it was only at the High Court — the final court of appeal in Australia — that all agreed his mistake was one of law, not fact.
The renewed emphasis on identifying the nature of the mistake could alter accepted wisdom about summary prosecutions for driving when suspended or disqualified. But more about that soon...