Come on Guya, some of these returns even a rocket scientist would have difficulty following. Be realistic. I give a good explanation to those that want one, but some will just never "get" it.
Yankeewannabe, if the IRS discovers a boo-boo or discrepancy, then they'll send you a letter. You take that letter to the guy or gal who did your return, and have them sort it out. Often times, it's the IRS who's at fault, not your preparer, so Do Not Reply; let them sort it out. They shouldn't charge you extra for this; they should guarantee their work. Most will charge you for representing you on an audit, although some have audit insurance programmes instead and a very few do it for free.
If it turns out that it's a stonking whopper of a mistake, they have professional indemnity insurance that covers the error. Usually, as you would have owed the tax anyways if they had done it "right", they pay your penalties only. Some also cover interest, but you had the right to your money in the meantime so some don't. It should be in the fine print of the client agreement documents you signed with your preparer.
Lastly, as to your responsibility to understand the tax law and return, Guya and Kitsonk are right: It is 100% your responsibility to be honest and truthful on your return. The preparer is likewise burdened with being 100% honest and truthful. If you tell your preparer that you spent $100 on chicken wire for your chicken coop, then (a) you had better have spent at least $100, and (b) your accountant had better not write a bigger number.
You may be surprised to learn that the IRS is kind and gentle provided you are truthful and honest in your dealings with them. "I didn't know she had to be in school full-time" is not an acceptable excuse for a mistake - either you or your accountant should have taken the time to know that. But "I lost the receipts in Hurricane Katrina and I'm not sure how much I spent" is acceptable, and while the IRS may still end up increasing the tax in such a case, they would generously waive all penalties for that person.