They're hoping it's not going to come up, but it always does. Then, they're upset. Agent Larry Underhill once had to inform a second wife that her husband had been previously married and owed court-ordered child support. The man had not disclosed those facts to his wife or on his mortgage application. I've had someone become disabled in a car wreck between the contract deal and the closing date.
People lose their jobs.
I had three homes under contract that were destroyed in the Haysville tornado. When that happens, you just help the buyers start over. Once, he took buyers into the wrong house. The surprised homeowners were gracious after Underhill introduced himself and apologized. Eventually, they called him to sell their home.