What is the defense of mistake of law?

Examples of this defense include the following:

a.  It is a complete defense to the crime of filing a false tax return that the accused had a good faith belief that the tax laws had been satisfied even if that belief was not reasonable.

b.  In another tax case, the accused—an attorney—relied on the “case-closed” accounting method for reporting his income.  Although this was not a valid accounting procedure, the appellate court deciding the case determined that the jury should have been instructed on the attorney’s “uncertainty of the law” defense.