The seller fails to get inspections a head of time so a buyer will find fault after contract
The seller overprices and when a buyer over pays the appraiser values the home less and the seller and buyer don't budge
The Buyer fails to get their lending in order prior to writing an offer
The Buyer fails to follow the rules during the contract and changes or quits job, over extends their credit
Or it could be a short sale and the seller's bank doesn't work with the parties.
Preparing the clients, keeping the end result in mind will result in a win-win.