We probably undercode sometimes and overcode sometimes. Is this common?

Coding mistakes are very common.  The incidence of miscoding approaches 50% in many estimates. Poor coding practices including undercoding may cost the average practice 10-20% of potential income. Overcoding subjects a practice to the risk of severe penalties.  The government has an active interest in pursuing wrongdoers through the Federal False Claims Act since it recovers $13 for every $1 expended for investigation.