I am wondering if anyone else has run into this scenario.  I have a Schedule C client for whom I entered the detail for the self employment tax deferral in the 2020 tax return.  However, the client did not qualify for the deferral because his tax refund was too large and there was nothing to defer.  The tax return as originally filed had correctly calculated 2021 suggested estimated tax based on the fact that this deferral was not included.  When I went back to the 2020 file months later during the 2021 filing season, a subsequent update must have triggered the estimated tax calculation to take into account the non-existent deferral.  I noticed that the 2021 estimated tax voucher amounts in the file had decreased by the potential deferred amount.  This seems to be a bug and thankfully only affected one client.  I think that there should be a way to "lock" the file after the tax return has been filed in order to prevent any changes to the file after the fact without manually "unlocking" it.