If you are looking to get out of a contract, do consult with your attorney--there are many factors that could kill a deal such as construction defects, home inspections in general, inability to obtain financing, appraisals, repairs that were agreed to be done weren't done, builder delays, mold, flooding, permits/c/o's, etc.
Your question is rather wide open - there are many things that can cause a transaction in New York State (or any state for that matter) to fail. The biggest and most lethal is the buyer's inability to get approved for a mortgage. If a house doesn't appraise for the agreed to price, usually gives the buyer an opportunity to either renegotiate the deal or back out. The lack of property certificates of occupancy can also kill a deal. Any serious problems occurring after the contract is signed such as floods, trees hitting the house, fires, etc. can also be a problem. If you're talking about what can stop a house from selling in the first place, it always comes down to one thing: price.
Certified Buyer Representative
Senior Real Estate Specialist
Century 21 Princeton Properties
Top 2% of Century 21 Agents Nationwide