The fact is 411A, that according to studies produced by several reliable scientific sources, the symptoms of fatigue are quite like the symptoms of inebriation, and are just as insidious. Your level of performance goes down at the same time your ability to self monitor and critique does.
You do not recognise that you are doing sh*te, on the contrary you feel that things are all hunky dory, hence no need to change the plan, all is "looking good" until the crunch.