| If you failed to indicate that you had a retirement plan and you did not make an IRA contribution, generally speaking, there should not be any additional tax penalties or interest that would be payable on your part.
If however, you failed to indicate that you had a retirement plan and you did make an IRA contribution, then the IRS will automatically adjust your tax return. However, if you fail to amend your tax return immediately the IRS determine the additional tax liability and assess you interest on the additional tax liability.
As a matter of fact, I actually encountered a similar situation in 2005. I filed the tax return and did not indicate that the client was covered with a 401k pension plan. Despite that, the client made an IRA contribution. Within 6 months the IRS responded by disallowing the IRA deduction, and the IRS assessed additional interest to the taxpayer. Clearly, the client was made to pay the interest on the additional tax liability.
Therefore, if your situation involves the fact that you omitted to include the fact that you were covered by a 401k pension plan and that you claimed an IRA contribution deduction, then I urge you to file an amended tax return and pay the additional tax liability. In this way, you would avoid paying the possible interest and or penalties associated with this additional tax liabilities. | 
