The mirror image rule is a rule in contract law that states that a contract is only valid when one party agrees to the exact terms of the contract. Whether or not a party does agree to the exact terms of the contract has heavy legal implications if a lawsuit arises.
If a party agrees to some parts of the contract, but not other parts, then this is considered a counteroffer. Unless they agree to the full contract terms, the contract will not be legally binding.
People who sign contracts should be very careful not to accept them unless they agree with every term of the contract. Once they sign, they will be obligated to adhere to the terms, or else potentially face a breach of contract suit.