Explanation:

Mistake of Only One Party:
When there is a mistake of only one party, the contract is considered valid. This is stated in Section 22 of the Indian Contract Act, 1872. Let's understand this in detail.

Section 22 of the Indian Contract Act, 1872:
According to Section 22, a contract is not voidable merely because one of the parties to the contract was under a mistake as to a matter of fact. In simple terms, if one party makes a mistake while entering into a contract, it does not render the contract voidable.

Example:
To better understand the concept, let's consider an example. A sells a painting to B, believing it to be a replica, while B believes it to be an original artwork. Later, it is discovered that the painting is indeed an original. In this case, A made a mistake regarding the nature of the painting, but B was aware of the true nature of the painting. Despite A's mistake, the contract between A and B will be considered valid.

Bilateral Mistake of Fact:
On the other hand, if both parties to a contract make a mistake regarding a material fact, the contract becomes void. This is mentioned in Section 20 of the Indian Contract Act, 1872.

Section 20 of the Indian Contract Act, 1872:
Section 20 states that a contract becomes void if both parties to the contract are under a mistake as to a matter of fact essential to the agreement. In such cases, the contract is considered void from the beginning, and the parties are relieved of their obligations under the contract.

Example:
Let's consider an example to illustrate this point. A agrees to sell his car to B, and both parties believe that the car has never been involved in an accident. However, it is later discovered that the car had been in a major accident before the sale. Since both parties made a bilateral mistake about a material fact (the car's accident history), the contract will be considered void.

Conclusion:
In summary, if there is a mistake of only one party in a contract, the contract is considered valid as per Section 22 of the Indian Contract Act, 1872. However, if there is a bilateral mistake of fact, the contract becomes void under Section 20.