The general answer: Your pattern is a single point of failure. If someone figures it out they can figure out your passwords to multiple sites. The most likely

The general answer: Your pattern is a single point of failure. If someone figures it out they can figure out your passwords to multiple sites. The most likely threat is your password to one site leaks or gets fished and then the hackers recognizes it as pattern generated and reverse engineers the pattern. Then they just start trying that pattern on other sites. Not doing this is good general advice. Your specific risk level varies based on the pattern you use.