4chan

I think there are plenty of failsafe mechanisms. But most importantly, if you fail to pay your mortgage, the bank has the right to take possession of your house. Those forces the bank to do it's due diligence with regards to the value of the house. Also, if a bank has been too lenient with its mortgages, it can get into serious trouble - the government here enforces pretty strict rules to prevent people from getting in over their heads.