In an article in The Toronto Star Murray Whyte writes that Mark Crispin Miller has noticed a disturbing pattern in Bush's frequent problems with speaking coherently.
In researching a book about Bushisms which was intended to be funny, Miller noticed that Bush sometimes is able to speak quite clearly and without grammatical problems, but other times cannot seem to remember simple, common phrases. He is able to speak clearly when he is talking about war, retribution, or other punitive, violent topics. On the other hand, "It's only when he leaps into the wild blue yonder of compassion, or idealism, or altruism, that he makes these hilarious mistakes." It is when Bush is speaking about topics that make him uneasy or with which he is uncomfortable that he has trouble speaking.
He comments specifically about the famous gaff involving the phrase "Fool me once, shame on you. Fool me twice, shame on me." What Bush actuall said when he tried to use this phrase was:
"Fool me once, shame ... shame on ... you." Long, uncomfortable pause. "Fool me — can't get fooled again!"
Miller compares this to an episode of Happy Days where the Fonz has to say "I'm sorry" and is unable to. This plays for laughs in a sitcom. If Bush cannot say "Shame on me" because he cannot imagine any shame ever attaching to himself, then this could indicate some underlying, unrealistic understandings of himself and his abilities.
As I've seen people make comparisons between Dubya and Hitler, there has always been a sense that the similarities in their actions and rises to power were purely coincidental. If Bush's misstatements are truly an indication of a sociopathic personality, as Miller suggests, then the comparisons seem much more frightening to me.