At one job, I was offered a promotion and took it. I became a supervisor and got a raise.
The owners of the company (it was a small family owned company) were asked by a friend, as favor, to hire his son for a year, while the son studied for a grad school exam. So basically make up a job for him and pay him for a year. He was fresh out of college. I was 15 years out of college and had a Master's degree.
Imagine my surprise when I found out that he was getting paid $4,000 more a year than I was.
Part of it was a big change in the job market. When I had been hired 7 years previously, the economy was bad and it was hard to get a job. When the kid was hired, things had changed and it was hard to find qualified employees, and as a result, salaries were higher.
While I debated for a week on how to handle this situation, the owner called me in and gave me a $5,000 raise.
And I earned every single penny of it, supervising that kid, who was one of the most incompetent human beings I have ever encountered. I spent that year re-doing pretty much everything he did, to get it to meet minimal quality standards.