What are the exceptions to the parol evidence rule? Which of the following is correct?

The parol evidence rule may not apply where one of the parties can demonstrate that the terms have caused some disadvantage to themselvesThe parol evidence rule will be discarded if it can be demonstrated that there were lengthy discussions taking place before the contract was written upThe exceptions are that there could be customary rules not referred to, that the contract was not entirely in writing or even that some of the words are ambiguous and need further clarification outside the written contractThe parol evidence rule is only a guide; a court in modern times can always look behind the agreement to determine what should be the 'equality of the bargain'