Sally goes to Honest Harry's used car lot to purchase a car. After test driving the car, she sits down to negotiate the contract. She asks about a warranty and Harry says that all cars that he sells come with a 30 day unconditional warranty on all parts and labor. She signs the contract and the next day the engine blows a rod. When she calls Harry she's told that there is no warranty according to the contract. She reads her contract and it says "as is"

Sally can do very little in this case due to the parol evidence rule. In common law, the parol evidence rule limits what type of evidence one party can use in a court in order to support their arguments. In this case, Sally cannot present the evidence of an oral contract because it was previous to the signing of the written contract. This rule doesn't allow certain older evidence to be presented in order to support a change of an existing contract.

I.e. an individual cannot try to change the terms of a contact once they have been signed just because in older contracts certain parts were different.