What Bugs says absolutely rings true. In general the purchase contracts that you enter into with the builder are quite restrictive. Many people who end up backing out because of cold feet or the shock of reality about the loan payment they are about to undertake do indeed lose their deposit. However if you can prove hardship of some type the lender may let you out of it. Furthermore if you really kick up and make a fight out of it and get an attorney then in all likelihood the lender will back off.
Once again, read your purchase contract BEFORE you sign it. Take the contract and bring it to an attorney if you like or even a Realtor.
You can even ask the salespeople your question you posed here and it would be interesting to see the response they give you. My bet is that they would give a blank look or say something like, “well that has never happened, you will not need to worry about that…”