The problem is that it never even gets to a serious decision between items under test. When before the test the testee (moron?) says, the difference is night and day, I could tell any time anywhere. In fact it is so easy my 8 year old can tell a difference. You have to be deaf not to hear it. Then after the blind test is failed miserably, the excuses start.
A "few" people have stated publicly (Amir and myself and others) in this great site that they were tuning in a stereo or subwoofer or both and after hours of tinkering and getting it just right, you find out the sub was not connected or some other issue that shows you tuned nothing. You ears and brain, well technically just your brain was fooling you the entire time.
The best thing to do is to walk away having learned just how fallible our hearing is. It is when the person gets quiet, acts like nothing happened and a day later denies that it ever happened. and still believes they can hear a flea fart on a close miked cat recording. So, when people are going to lie like that, you really have nothing else to say except to stop engaging with that liar, I mean person, on that subject. Return to the old "which car is faster discussions" where his mind is free to wander without insulting your intelligence and ethics.
Just sayin