I would never use them but, according to H&R Block's standard guarantee, a person is covered if penalty and interest charges are assessed due to H&R Block's error, H&R Block will pay those penalties and interest on federal, state and local returns.
That being said unless you (or whomever) purchased the "Peace of Mind Extended Service Plan" then you (or whomever) will have to pay the actual extra monies due.
In short without the additional service plan they offer, you pay for the original underpaid amount and they pay for the penalties and interest incurred because of it.
Here-
http://www.hrblock.com/offices/tax-services-peace-of-mind-service-plan.html