It's funny how employees get to thinking they are irreplaceable. I have had this happen on several occasions and it's never a good thing.
It probably has to do with human nature. People want (or need) to feel important. I have no problem with that. The problem comes when they get to feeling too important. I like employees who can carry the ball. But I don't like it when they won't throw the ball back.
When an employee gets to feeling like he is the most important component in your business, your business begins to suffer. A good example is a guy I'll call Bob. Bob started out great. He was motivated and wanted to carry his weight. He quickly learned my methods and became very adept at putting things together they way I liked and he understood that I was the one calling the shots. Over time, Bob became a key figure in my shop and could be depended on to direct other employees.
But after a while, I noticed that Bob was tending to override my directions. He started thinking he had a better idea of how things should be done than I had. I noticed this when I asked him to set up a shaper run and it was not done how I had asked him to do it. He told me, "I figured it would be easier this way."
This trend continued to the point where Bob was saying things like "imagine that" when I pointed out that he was not following my directions. At that point, I realized Bob's thinking had gone south. He had the idea that he knew much better than I how to run my shop. And since Bob had become something of a manager, the other employees were beginning to second guess me as well.
We had a lot of conversations about this, which just seemed to make things worse. He had the idea that he was an irreplaceable employee and that my shop could not function without him. Finally, after a particularly defiant episode, I told him to go.
The funny thing is, even after all of the warnings and conversations we had, he was genuinely shocked. He became extremely agitated and upset, and all I could think of to say was "imagine that."