To be frank, the main consequence would be simply not achieving that goal. Setting an unrealistic goal means that it's probably pretty hard to achieve, but you may not see it that way, so when you eventually fail or mess up, like any normal person would, you will be extremely discouraged, when in reality there is no reason to be disappointed in yourself given the goal was unattainable in the first place. You may feel inferior and it might lower your self esteem because you thought that the goal should have been achieve able.