This isn't his first “numeracy” mistake. Some of them are orders of magnitude wrong.
There are thousands of reason to dump on Obama, but him saying “57” instead of “47” is not one of them. Pretending common verbal stumbles are a sign of something serious makes it look like he has no real problems.
This thread is juvenile bull-sh~t. Have fun with it if you like, but don’t be fooled into thinking it’s any more seriously than a swimming pool fart game.
http://www.bspcn.com/2007/11/11/the-30-dumbest-things-george-w-bush-has-ever-said/