I know it seems like Bush forgot the quote, but in reality he likely realized halfway through he didn't want a soundbite of him saying "Shame on me".
Bush tripped over his words all the time. It's much more likely that than a calculated move (that backfired btw, it's been his most famous quote since the day he said it.)
I remember hearing people suggest this is why he misspoke shortly after it happened.
Of all people to give the benefit of the doubt to...you give it to a bumbling idiot who was known for misspeaking and tongue tying himself into knots?
Before he even messed up the saying, he messed up his own introduction to the saying:
>There's an old saying in Tennessee — I know it's in Texas, probably in Tennessee...