10. Mike Singletary
In 2008, Singletary became interim head coach of the 49ers. He impressed with his charisma. After turning results around to finish 5-4 under his leadership, they awarded him a four-year contract. This is where they should have said thanks for everything and brought in somebody with more experience. Instead, they chose to welcome him to San Francisco.
The 49ers paid the price for being sentimental. The results rapidly went downhill. They finished 8-8, breaking even in his first full season. Then results totally plummeted the following year with a miserable record of 5-10. The 49ers finally fired him in the last week. Tactically limited, his aggressive demeanor rubbed off the wrong way on his players.