The errors occurred from January 2003 to April 2004. During that time, the test - the Praxis Principles of Learning and Teaching for Grades 7 to 12, called the Praxis P.L.T. 7-12 - was given eight times, to a total of about 40,000 people.
That means ten percent of these potential teachers were erroneously failed.
But what's far more important, and naturally completely unquestioned by this article, is how one's general ability to teach can be measured by filling in scantron forms and writing short responses on a standardized test. I have no problem with testing to make sure potential teachers are competent in the subject matter which they want to be teaching -- history teachers should know dates and names, physics teachers should know the right formulae, gym teachers should know how to blow a whistle and order pushups. But the ability to manage a classroom and convey knowledge effectively is more akin to being able to perform on stage, and composed of so many distinct factors that it strikes me as absurd or naive or both to think it can be expressed as a single number, which is then compared to an arbitrary cutoff to determine whether the individual is to be allowed to teach or not. The only way to determine whether an individual is a qualified teacher is to put them in a classroom and watch them.