Once the parties sign the real estate purchase agreement, they are "under contract" to fulfill certain duties before a closing date. What are some of the reasons that the actual purchase of the real estate under one of these contracts (i.e. the "closing") might not occur?

For each of these reasons, who would be the more likely party to break the contract and why?

For each of these reasons, are they able to break the contract without recourse or penalty, and if not, what is the penalty?