“The president of a company called “the Blue Waters Group” told WSBT-TV the mistake was not made by the city or the schools. He said the ad was proofed by 4 people and the typo got by all of them.” source
I never made that mistake but it was only by the Grace of God…it is a most common print typo. It is easy to do and hard to catch.
Here is a story about two guys who went on the great typo hunt, scouring public signage across America to find the most common typos. Good story.