What was there to find. Anyone with half a brain spotted that right away. His most important speech to date, a speech that may be repeated for years to come. It is a pretty glaring mistake.
BTW, we should also give a prize to the person that used a magnifying glass to find the mistake in this quote: "Over the last 15 months, we’ve traveled to every corner of the United States. I’ve now been in 57 states? I think one left to go."