What can go wrong between exchange and completion

What happens between exchange and completion and what could possibly go wrong? In England and Wales the penultimate stage of a property sale is exchange of contracts. This is the point at which both the buyer and the seller are committed to the sale and purchase. This is usually when 10% of the purchase price is paid by the buyer. At this point neither party can withdraw without having to pay damages to the other party. The final stage of a property sale is completion.