That's "a spelling error".
I blew twenty-four mod points, came home from work, crashed my car, paid a thousand dollars, screamed at some people on the street, and made my girlfriend break up with me in order to fix that missing article.
First part of your comment was good, even funny. Your tagline, "I've taught at both union and non-union schools. Unions are better for students and teachers." not so much. All 'public (dis)service' unions, and that includes the teachers' unions -- which are the proximate cause of the demonstrable stupidity of the last two generations -- are simply extortionate groups preying on the public purse.