What mistakes cause business owners to overpay in taxes?

Mistakes most often come from things you are not doing rather than things you are doing wrong. Your bookkeeping and accounting should catch the potential mistakes in things you are doing. A financial advisor should help with finding the things you are not doing.
Some recent ones we have seen involve retirement plan contributions, profit-sharing, end-of-year expenses, and poor bookkeeping. We have also seen gaps in communication between accountant, client, and prior financial advisor.
Most of the time, the planning ideas are easy to come by. The challenge comes in finding which is best for your unique situation along with communicating the plan and implementing it with the other people on the team.