If you told them porkies on your application form and they found out then I'm sure you would lose your approval.
If you change jobs they don't like that much either - you have to complete your probationary period. Lenders think you're too high a risk on probation because you might not be made permanent and wouldn't then be able to keep up payments.
Your loan is also dependent on the lender thinking that the house your to buy is also worth the money they'll lend you. If the house is in poor condition, they may refuse you a mortgage (or slap on a load of conditions to do with required improvements), because the resale value of the house is their own cast-iron guarantee of getting their money back.
The loan may also be substantially reduced if you want to buy what the lender views as a 'specialist property.' Many lenders won't give more than 60%-75% for such places in order to reduce their exposure to subsequent problems with such houses or a difficult resale.