The most frequent, costly errors are: using the wrong HS code because a product was matched on a rough description rather than its actual construction and function; undervaluing an invoice, whether deliberately or through a genuine misunderstanding of what must be included; forgetting to add royalties, licence fees or assist costs to the customs value; and failing to secure a valid certificate of origin before the goods ship, when it is far harder to obtain retroactively.
Any of these can trigger a customs audit, back-duty assessment, or penalty long after the shipment has already been sold, so build classification and valuation checks into your process before you place an order, not after a problem appears.