Avoid fines and problems with the tax authority by knowing these common mistakes. Mistakes to Avoid When Dealing with Taxes Many business owners face tax problems due to lack of knowledge or negligence. Here are the most common mistakes: Failure to file tax returns on time. Neglecting regular bookkeeping. Not registering for VAT when reaching the registration threshold. Mixing personal and business expenses. To avoid these issues, we always recommend consulting an experienced certified accountant.