Ultimately the employee has to make sure that the amount contributed to the HSA is within federal guidelines. However, employers should have ways to keep employees from deducting more than the maximum HSA contribution amount. The custodian (Optum Financial) cannot accept more than the family HSA limit plus the catch-up contribution.
HSA contributions that go over the IRS annual contribution limits are not tax deductible and employees generally pay a 6% excise tax.