My wife recently applied to a new job that would bring in better benefits and a significant amount of more money. The company required several references, one of which had to be a previous supervisor or manager. She's only had the same manager for the last 5 years so that's the person she pretty much was forced to use but this person has a history of terrible behavior. That person has called her on off hours completely drunk harassing her. That person has misused her PTO and sick time. That person has threatened her job one week and praised her for being the best employee the next.
That person gave my wife an absolutely appalling review and the new job rejected her because of it. The goal was to keep my wife around because they knew that they were getting exceptional work out of her with minimal pay. They purposely bombed the review in hopes that she wouldn't get the new job and would have to stay.