Mistakes of fact which affect the validity of a contract are referred to as operative mistakes. The law recognizes various types of operative mistakes for example:
Common or shared mistake: this is a mistake as to the existence or ownership of the subject matter. Both parties make the same mistake. Each party appreciates the others intention and accepts the same, but both are mistaken about an underlying fundamental fact. Common mistake renders a contract void in two circumstances.
o Cases of res extincta: the subject matter does not exist. This scenario is embodied in section 8 of the Sale of Goods Act, Cap 31. It is illustrated by inter alia the decision in Couturier V. Hastie.
o Cases of res sua: the subject matter belongs to the party purporting to purchase the same (Bingham V. Bingham).
Mutual Mistake: this is a mistake as to the subject matter. The parties are at cross purposes or misunderstand each other. There is no agreement between them for want of Consensus ad idem. The purported agreement is void. The decision in Raffles V.Wichelhouse illustrates this principle.
Unilateral mistake: this is a mistake as to the identity of the other party to the contract.
It arises in circumstances in which a fraudulent person misrepresents his identity so as to obtain goods on credit or other favourable terms which he promptly sells to an
unsuspecting third party or a bonafide purchaser. The dispute is ordinarily between the original seller and the bonafide purchaser. The goods or their value are recoverable if the
original seller demonstrates that the contract between him and the fraudulent person was void for unilateral mistake. It must be evident that:
o The seller dealt with a person other than the person intended.
o The person dealt with was aware of this fact.
o The identity of the person the seller intended to deal with was fundamental to the contract.
The decision in Cundy V. Lindsay and Co. exemplifies this type of mistake.
Mistakenly signed documents: this is a mistake as to the nature of the contract i.e. signing the wrong contract. Such a mistake renders the contract voidable at the option of the innocent party.
Mistake as to quality of subject matter: such a mistake renders the contract voidable at the option of the innocent party.
Makosa ya ukweli ambayo huathiri uhalali wa mkataba hujulikana kama makosa ya uendeshaji. Sheria inatambua aina mbalimbali za makosa ya uendeshaji, kwa mfano, kosa la kawaida au la pamoja: hii ni kosa kuhusu kuwepo au umiliki wa mada. Pande zote mbili hufanya kosa lilelile. Kila upande unathamini nia ya mwingine na unakubali sawa, lakini wote wawili wanakosea juu ya ukweli wa msingi wa msingi. Kosa la kawaida hufanya mkataba usiwe na maana katika hali mbili. o Kesi za res extincta: jambo hilo halipo. Hii ni kwa mujibu wa kifungu cha 8 cha Sheria ya Uuzaji wa Bidhaa (Cap 31). Ni kuonyeshwa na, miongoni mwa mambo mengine, uamuzi katika Couturier V. Hastie. o Kesi ya res sua: jambo hilo ni mali ya chama kinachojitahidi kununua sawa (Bingham V. Bingham). Kosa la Pamoja: hii ni kosa kuhusu mada. Vyama viko katika makusudi ya msalaba au huelewana vibaya. Hakuna makubaliano kati yao kwa sababu ya kukosa makubaliano ad idem. Mkataba unaodaiwa kuwa huo haufai. Uamuzi katika kesi ya Raffles v. Wichelhouse unaonyesha kanuni hiyo. Kosa la upande mmoja: hii ni kosa kuhusu utambulisho wa chama kingine cha mkataba. Hii hutokea katika hali ambapo mtu wa udanganyifu anajitokeza kwa njia isiyo sahihi ili kupata bidhaa kwa mkopo au masharti mengine mazuri ambayo yeye huuza kwa haraka kwa mtu wa tatu asiye na shaka au mnunuzi wa kweli. Mzozo ni kawaida kati ya muuzaji wa awali na mnunuzi wa bonafide. Bidhaa au thamani yake inaweza kurejeshwa ikiwa muuzaji wa awali anaonyesha kwamba mkataba kati yake na mtu wa udanganyifu ulikuwa haujakuwa na maana kwa kosa la upande mmoja. Ni lazima iwe wazi kwamba: o Mfanyabiashara alifanya biashara na mtu mwingine kuliko mtu aliyekusudiwa. o Mtu aliyeshughulikiwa alikuwa na ufahamu wa ukweli huu. o Utambulisho wa mtu muuzaji alikusudia kushughulika na ilikuwa ya msingi kwa mkataba. Uamuzi katika Cundy V. Lindsay na Co. ni mfano wa aina hii ya kosa. Hati zilizosainiwa kwa makosa: hii ni kosa kuhusu asili ya mkataba yaani Kujiunga na mkataba usiofaa Kosa kama hilo hufanya mkataba huo usiwe na maana kwa hiari ya mtu asiye na hatia. Kosa kuhusu ubora wa mada: kosa kama hilo hufanya mkataba usiwe na maana kwa chaguo la chama kisicho na hatia.