My wife is a teacher. She wanted to teach history, so she had to get a history degree with a specialization in education. But there were no jobs available, so she accepted a conditional as a special education teacher. That's what drove her to get a master's degree in special ed.

While doing teaching special ed she developed a fondness for teaching math. But she isn't allowed to take on a general ed math class because she doesn't have a "math endorsement" - which would require her to go back to school again for basically another advanced degree in math. And she can't get a general ed job in history because it's too competitive and her years of experience makes her too expensive compared to fresh blood.