I think a huge mistake businesses make is focusing too much on what their competition is doing, instead of focusing their efforts narrowly on knowing their customers better and providing them exact what they want to buy and pay money for. The competition isn't always right. They may be less informed than you are. Don't follow them off a cliff for no reason. Focus on giving your customers what they want.
I think two of the big ones are not keeping an eye on the competition and an unwillingness to fire a bad client. When you don't watch the competition, you might miss the fact that the market is moving in a new direction or that a new opportunity is growing. You get stuck in what you are doing instead of growing. Failing to fire a bad client is similar. You get bogged down spending all your energy on customers who are not bringing in profits proportionate with the effort they demand and you miss the chance to grow and serve more positive customers.