Have you ever been in a home improvement store and noticed the shelf of mixed paint that is sitting there discounted? It is usually oops paint. When the paint got mixed it ended up not being the exact color that the person was looking for. Because the original customer doesn't want it, it can be a huge savings for you.
If you have a large area to paint, oops paint probably isn't your best choice, but if you are dong an accent wall or a small area it may be just enough. We painted a half bath in a previous house with a can of oops paint. Steve was smart enough to save the area behind the mirror for last. That was great because he ran out just when he got to that area. I am apologize to the new owners in case they have decided to change the mirror and now realize they have to paint too. You will have a hard time finding that exact color.