Say an active employee discovers an error in his accrued pension. What remedy is there? Could the employee file a formal claim for additional benefits, even though he has not even commenced benefits? Or is this handled more informally through letters?

Active employee? Where is the harm? Where is the need to file a claim?

If we are focusing on an error, that usually means (to me anyway) a data correction. If so, the employee should bring that to the attention of the plan sponsor. But beware, sometimes a data correction might not affect the benefit. Plan provisions still apply.

If the "error" is related to an interpretation of plan provisions, then my advice is unchanged: bring it to the attention of the employer, and ask for a response.

In any case, the employer should respond with its analysis of the facts and/or plan interpretation.