Contingencies are the big way a deal can fall through.
Inspection Contingency: The inspector (or specialists, if it goes that far) finds something horribly wrong and/or expensive to fix or finds too many things that the buyer backs out (or negotiations on a new price and repairs fail).
Appraisal Contingency: The house doesn’t appraise high enough and the bank won’t provide enough financing for the buyer to purchase the house. The seller and buyer have to come up with a new price or the buyer has to come up with the difference in price.
HOA Contingency: If there is an HOA, often there is language in the contract that allows you to back out after reviewing the HOA documents (financials, bylaws, dues, etc…) if you’re not comfortable with that HOA and the way it appears to be run. Check with your agent about this if you’re unsure if you have that out and what options you have.
This list is great. I’d add financing contingency as well. Buyers messing up their loan process is not super common but definitely happens when folks are rushed and don’t vet their finances correctly or are stretched very thin and any mistake during the loan process can mess up their numbers. Also, government-backed loans like VA and FHA have their own set of requirements that if the buyer is unaware, can delay or potentially kill a deal.
As a seller, one way to get a better idea of the quality of a Pre-approval letter is to call the loan officer on the letter directly and ask if they have received and reviewed income and assets.
Unfortunately, not all lenders follow the same process. As a loan officer, I am very careful about ensuring a buyer is properly qualified prior to make an offer. I hear horror stories all the time about this. I hope you are able to get a good buyer for your property! If you need help in knowing what to ask to ensure the pre-approval is good I would be happy to help.