Interesting analysis from here:

But equating Bush's malapropisms with Quayle's inability to spell "potato" is a dangerous assumption, Miller says.

At a public address in Nashville, Tenn., in September, Bush provided one of his most memorable stumbles. Trying to give strength to his case that Saddam Hussein had already deceived the West concerning his store of weapons, Bush was scripted to offer an old saying: Fool me once, shame on you. Fool me twice, shame on me. What came out was the following:

"Fool me once, shame ... shame on ... you." Long, uncomfortable pause. "Fool me — can't get fooled again!"

Played for laughs everywhere, Miller saw a darkness underlying the gaffe.

"There's an episode of Happy Days, where The Fonz has to say, `I'm sorry' and can't do it. Same thing," Miller said.

"What's revealing about this is that Bush could not say, `Shame on me' to save his life. That's a completely alien idea to him. This is a guy who is absolutely proud of his own inflexibility and rectitude."

I still don't think Bush is any threat to Einstein, but this would explain a lot.