The mistake was not essential since the subject matter of the contract was in essence the service agreements and the fact that they might have been terminated without compensation was only a side issue.
Error in substantia (Roman law): if the thing contracted for was essentially different in quality or attributes from the thing as it was believed by both parties to be the contract would be void.
Clearly not 119 II, but reclaimable in 812ff. See Spanish rules on 'unjust enrichment' - no claim without erroneous payment.

A took a lease of a fishery from B.
It transpired (unknown to both parties) that A was really the owner of the fishery.

This was a case in equity, though the result would have been similar at common law.
Where equity does grant relief it will impose its own terms.
Mistake as to title : mistake of law will never avoid a contract, but a common mistake as to private title may.

Corn was sold while in transit by sea from Salonica to England.
It transpired that at the time of the contract the ship's master had in fact disposed of the corn at Tunis.

If at the time when the contract is made, unknown to both parties, the subject matter of it does not exist the contract will be void.
This was a mistake as to the existence of the subject matter.

A fraudulent person ordered goods pretending to be another person.
The goods he then sold to a third party.

The plaintiffs had a specific, existing person in mind.

A person presenting himself with a different but existing name bought a car from the plaintiff.
The plaintiff checked the name in the phone book.
The defendent paid with a check, which bounced.

A rogue called Wallis pretended to be an imaginary firm which he called 'Hallam & Co.' and had pretentious notepaper bearing that name printed.
He ordered goods from the plaintiffs by writing to them on this notepaper and they sent the goods to him.
He then sold the goods to the defendant.

Wallis' personality could not have affected the minds of the plaintiffs - if they were willing to give credit to 'Hallam & Co.', a non-existent entity, they were willing to give it to anyone.
Though there was fraud, there was no operative mistake.

A picture believed by both parties to be a Constable was sold by the one to the other.
It turned out not to be a Constable.
The the buyer sued.

The plaintiff sold a car to a man who presented himself to be Richard Greene (a famous actor).

The defendants, having disposed to the plaintiffs of a wreck believed by both parties to be lying on a named reef.
When in truth not only the wreck but also the reed were non-existent.

The situation was such that the defendants must have been taken, in view of the very nature of their business, to have warranted the existence of the thing they purported to "dispose" of.
It is possible for a person to contract upon the terms that he will be liable whether or not the subject matter exists.
Such a term can be implied.

A jeweller made a sale to one 'Sir George Bullough', who was in fact a man called North.
The jeweller later sued a third innocent party.

W agreed to buy cotton to arrive on the Peerless sailing from Bombay.
W intended the ship sailing from Bombay in October.
R offered the cotton from another ship sailing in December (both names the same).

The defendant meant one ship and the plaintiff another.

A landlord mistakenly inserted in a lease a lower rent than he had intended to demand.

Equity follows the law; it is to correct its defects, not to take its place.

An old lady, who had broken her spectacles, was persuaded to assign the lease of her house.
With the result that a companion of her nephew's was enabled to raise a loan from a building society upon the security of it.

Her mistake had been that she was transferring the house by the way of gift to the nephew in order to help him to secure a loan.
Whereas in fact she had been persuaded to make a purported sale to the companion who, having obtained the loan from the Society, absconded with the money.
It was made plain that a prima facie burden lies in such a case upon the signer establish that in signing as he did he was not negligent. Hence, those who sign things in reliance upon the statements of others without reading them, can seldom be protected by the plea.

It was agreed that one of the parties should deliver to the other a quantity of sisal growing on a certain piece of land which (unknown to both parties) was incapable of producing he stipulated amount of sisal.

Mistake as to physical possibility: both parties think that they are contracting about something which is physically possible the contract will be void if what is contracted for turns out to have been a physical impossibility.

No one can escape his obligations simply by stating that he was mistaken about the terms of the contract.
He can only escape if he proves that he was mistaken, and even then the mistake must be one which relates to some material particular.

Otherwise all contractual transactions would be in danger of being set aside at the whim of a party who alleges that he did not appreciate the nature of his undertaking.

The landlord and tenant wrongly thought that the house let was outside the Rent Act limits, whereas in fact it was within

In equity, a contract may be voidable (not void, thus leaving room for the protection of third party rights) where the mistake is one related to the attributes rather than the essence of the subject matter.
The grant of equitable remedies (rescission, in this case) is discretionary and in exercising this discretion the courts will take the effect of mistake into account upon grounds wider than the grounds recognised by the common law.