From Black's Law Dictionary:
Mistake (criminal law)
Mistake (contract law) From Wikipedia
Mistake of law and mistake of fact are two types of defense by
excuse, via which a defendant may argue that they should not be held criminally
liable for breaking the law or liable for damages under a civil law action.
Most criminal courts do not recognize mistake of law, in which the defendant
argues that they never knew of the law and thus should not be held liable. This
defense is often countered with the clichéd maxim: "Ignorance of the
law is no excuse."
Mistake of fact is sometimes seen as valid. For example, if one were to go
to an airport and pick up a bag which looked like one's own, and that bag were
to contain a bomb, one might argue that a mistake had led to possession of the
bomb. Another common example is taking another person's coat from a coatrack when
you intended to take your own. This can also be interpreted as the lack of a mens
rea. For a leading Supreme Court of Canada case on the mistaken belief defence
see: R. v. Park.
In contract law a mistake is incorrect understanding by one or
more parties to a contract and may be used as grounds to invalidate the agreement.
Common law has identified three different types of mistake in contract: unilateral
mistake, mutual mistake, and common mistake.
A unilateral mistake is where only one party to a contract is mistaken as to the
terms or subject-matter. The courts will uphold such a contract unless it was
determined that the non-mistaken party was aware of the mistake and tried to take
advantage of the mistake.
The leading case on unilateral mistake is Smith
Mistake of identity
It is also possible for a contract to be voidable if there was a mistake in the
identity of the contracting party. In the leading English case of Lewis v Avery
 3 All ER 907 Lord Denning held that the contract can only be avoided if
the plaintiff can show, that at the time of agreement, the plaintiff believed
the other party's identity was of vital importance. A mere mistaken belief as
to the credibility of the other party is not sufficient.
A mutual mistake is when both parties of a contract are mistaken as to the terms.
Each believe they are contracting to something different. The court usually tries
to uphold such a mistake if a reasonable interpretation of the terms can be found.
The famous case of the Peerless ship is an example in the case of Raffles v.
Wichelhaus, (1864) 2 Hurl. & C. 906. The defendant had made an order for the
purchase of cotton for goods arriving on a certain boat Peerless from Bombay leaving
in October. However a different boat arrived called Peerless, also from Bombay,
but having left in December. The plaintiff merchant sought to enforce the contract
for the sale of cotton, but the defendant refused stating that it was not the
cotton that he had ordered. The court stated that reasonable meaning must be found.
However, on the facts, there was no single reasonable interpretation of the terms,
both parties were equally mistaken, thus the contract was void.
A common mistake is where both parties hold the same mistaken belief of the facts.
The House of Lords case of Bell v. Lever Brothers Ltd. established that common
mistake can only void a contract if the mistake of the subject-matter was sufficiently
fundamental to render its identity different from what was contracted, making
the performance of the contract impossible.
Later in Solle v. Butcher, Lord Denning added requirements for common mistake
in equity, which loosened the requirements to show common mistake. However, since
that time the case has been heavily criticized in cases such as Great Peace.