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Is is possible to dispute IRS underpayment penalties?
didn't pay the extra underpayment penalty on the grounds that it was   an honest mistake. You seem to think a penalty applies only when the IRS thinks you were trying to cheat the system.  That's not the case. A mistake (honest or otherwise) still can imply a penalty. While you can appeal just about anything, on any grounds you like, it's unlikely you will prevail.